From 8ac6c7a54ed1b98d142dce24b11c6de6a1e239a5 Mon Sep 17 00:00:00 2001
From: hc <hc@nodka.com>
Date: Tue, 22 Oct 2024 10:36:11 +0000
Subject: [PATCH] 修改4g拨号为QMI,需要在系统里后台执行quectel-CM
---
kernel/Documentation/vm/hmm.rst | 401 ++++++++++++++++++++++++++++++++-------------------------
1 files changed, 225 insertions(+), 176 deletions(-)
diff --git a/kernel/Documentation/vm/hmm.rst b/kernel/Documentation/vm/hmm.rst
index cdf3911..09e2850 100644
--- a/kernel/Documentation/vm/hmm.rst
+++ b/kernel/Documentation/vm/hmm.rst
@@ -1,4 +1,4 @@
-.. hmm:
+.. _hmm:
=====================================
Heterogeneous Memory Management (HMM)
@@ -10,7 +10,7 @@
this document).
HMM also provides optional helpers for SVM (Share Virtual Memory), i.e.,
-allowing a device to transparently access program address coherently with
+allowing a device to transparently access program addresses coherently with
the CPU meaning that any valid pointer on the CPU is also a valid pointer
for the device. This is becoming mandatory to simplify the use of advanced
heterogeneous computing where GPU, DSP, or FPGA are used to perform various
@@ -22,8 +22,8 @@
section gives an overview of the HMM design. The fourth section explains how
CPU page-table mirroring works and the purpose of HMM in this context. The
fifth section deals with how device memory is represented inside the kernel.
-Finally, the last section presents a new migration helper that allows lever-
-aging the device DMA engine.
+Finally, the last section presents a new migration helper that allows
+leveraging the device DMA engine.
.. contents:: :local:
@@ -39,20 +39,20 @@
i.e., one in which any application memory region can be used by a device
transparently.
-Split address space happens because device can only access memory allocated
-through device specific API. This implies that all memory objects in a program
+Split address space happens because devices can only access memory allocated
+through a device specific API. This implies that all memory objects in a program
are not equal from the device point of view which complicates large programs
that rely on a wide set of libraries.
-Concretely this means that code that wants to leverage devices like GPUs needs
-to copy object between generically allocated memory (malloc, mmap private, mmap
+Concretely, this means that code that wants to leverage devices like GPUs needs
+to copy objects between generically allocated memory (malloc, mmap private, mmap
share) and memory allocated through the device driver API (this still ends up
with an mmap but of the device file).
For flat data sets (array, grid, image, ...) this isn't too hard to achieve but
-complex data sets (list, tree, ...) are hard to get right. Duplicating a
+for complex data sets (list, tree, ...) it's hard to get right. Duplicating a
complex data set needs to re-map all the pointer relations between each of its
-elements. This is error prone and program gets harder to debug because of the
+elements. This is error prone and programs get harder to debug because of the
duplicate data set and addresses.
Split address space also means that libraries cannot transparently use data
@@ -77,12 +77,12 @@
I/O buses cripple shared address spaces due to a few limitations. Most I/O
buses only allow basic memory access from device to main memory; even cache
-coherency is often optional. Access to device memory from CPU is even more
+coherency is often optional. Access to device memory from a CPU is even more
limited. More often than not, it is not cache coherent.
If we only consider the PCIE bus, then a device can access main memory (often
through an IOMMU) and be cache coherent with the CPUs. However, it only allows
-a limited set of atomic operations from device on main memory. This is worse
+a limited set of atomic operations from the device on main memory. This is worse
in the other direction: the CPU can only access a limited range of the device
memory and cannot perform atomic operations on it. Thus device memory cannot
be considered the same as regular memory from the kernel point of view.
@@ -93,20 +93,20 @@
order of magnitude higher latency than when the device accesses its own memory.
Some platforms are developing new I/O buses or additions/modifications to PCIE
-to address some of these limitations (OpenCAPI, CCIX). They mainly allow two-
-way cache coherency between CPU and device and allow all atomic operations the
+to address some of these limitations (OpenCAPI, CCIX). They mainly allow
+two-way cache coherency between CPU and device and allow all atomic operations the
architecture supports. Sadly, not all platforms are following this trend and
some major architectures are left without hardware solutions to these problems.
So for shared address space to make sense, not only must we allow devices to
access any memory but we must also permit any memory to be migrated to device
-memory while device is using it (blocking CPU access while it happens).
+memory while the device is using it (blocking CPU access while it happens).
Shared address space and migration
==================================
-HMM intends to provide two main features. First one is to share the address
+HMM intends to provide two main features. The first one is to share the address
space by duplicating the CPU page table in the device page table so the same
address points to the same physical memory for any valid main memory address in
the process address space.
@@ -121,14 +121,14 @@
hardware specific details to the device driver.
The second mechanism HMM provides is a new kind of ZONE_DEVICE memory that
-allows allocating a struct page for each page of the device memory. Those pages
+allows allocating a struct page for each page of device memory. Those pages
are special because the CPU cannot map them. However, they allow migrating
main memory to device memory using existing migration mechanisms and everything
-looks like a page is swapped out to disk from the CPU point of view. Using a
-struct page gives the easiest and cleanest integration with existing mm mech-
-anisms. Here again, HMM only provides helpers, first to hotplug new ZONE_DEVICE
+looks like a page that is swapped out to disk from the CPU point of view. Using a
+struct page gives the easiest and cleanest integration with existing mm
+mechanisms. Here again, HMM only provides helpers, first to hotplug new ZONE_DEVICE
memory for the device memory and second to perform migration. Policy decisions
-of what and when to migrate things is left to the device driver.
+of what and when to migrate is left to the device driver.
Note that any CPU access to a device page triggers a page fault and a migration
back to main memory. For example, when a page backing a given CPU address A is
@@ -136,8 +136,8 @@
address A triggers a page fault and initiates a migration back to main memory.
With these two features, HMM not only allows a device to mirror process address
-space and keeping both CPU and device page table synchronized, but also lever-
-ages device memory by migrating the part of the data set that is actively being
+space and keeps both CPU and device page tables synchronized, but also
+leverages device memory by migrating the part of the data set that is actively being
used by the device.
@@ -147,120 +147,117 @@
Address space mirroring's main objective is to allow duplication of a range of
CPU page table into a device page table; HMM helps keep both synchronized. A
device driver that wants to mirror a process address space must start with the
-registration of an hmm_mirror struct::
+registration of a mmu_interval_notifier::
- int hmm_mirror_register(struct hmm_mirror *mirror,
- struct mm_struct *mm);
- int hmm_mirror_register_locked(struct hmm_mirror *mirror,
- struct mm_struct *mm);
+ int mmu_interval_notifier_insert(struct mmu_interval_notifier *interval_sub,
+ struct mm_struct *mm, unsigned long start,
+ unsigned long length,
+ const struct mmu_interval_notifier_ops *ops);
-
-The locked variant is to be used when the driver is already holding mmap_sem
-of the mm in write mode. The mirror struct has a set of callbacks that are used
-to propagate CPU page tables::
-
- struct hmm_mirror_ops {
- /* sync_cpu_device_pagetables() - synchronize page tables
- *
- * @mirror: pointer to struct hmm_mirror
- * @update_type: type of update that occurred to the CPU page table
- * @start: virtual start address of the range to update
- * @end: virtual end address of the range to update
- *
- * This callback ultimately originates from mmu_notifiers when the CPU
- * page table is updated. The device driver must update its page table
- * in response to this callback. The update argument tells what action
- * to perform.
- *
- * The device driver must not return from this callback until the device
- * page tables are completely updated (TLBs flushed, etc); this is a
- * synchronous call.
- */
- void (*update)(struct hmm_mirror *mirror,
- enum hmm_update action,
- unsigned long start,
- unsigned long end);
- };
-
-The device driver must perform the update action to the range (mark range
-read only, or fully unmap, ...). The device must be done with the update before
-the driver callback returns.
+During the ops->invalidate() callback the device driver must perform the
+update action to the range (mark range read only, or fully unmap, etc.). The
+device must complete the update before the driver callback returns.
When the device driver wants to populate a range of virtual addresses, it can
-use either::
+use::
- int hmm_vma_get_pfns(struct vm_area_struct *vma,
- struct hmm_range *range,
- unsigned long start,
- unsigned long end,
- hmm_pfn_t *pfns);
- int hmm_vma_fault(struct vm_area_struct *vma,
- struct hmm_range *range,
- unsigned long start,
- unsigned long end,
- hmm_pfn_t *pfns,
- bool write,
- bool block);
+ int hmm_range_fault(struct hmm_range *range);
-The first one (hmm_vma_get_pfns()) will only fetch present CPU page table
-entries and will not trigger a page fault on missing or non-present entries.
-The second one does trigger a page fault on missing or read-only entry if the
-write parameter is true. Page faults use the generic mm page fault code path
-just like a CPU page fault.
+It will trigger a page fault on missing or read-only entries if write access is
+requested (see below). Page faults use the generic mm page fault code path just
+like a CPU page fault.
Both functions copy CPU page table entries into their pfns array argument. Each
entry in that array corresponds to an address in the virtual range. HMM
provides a set of flags to help the driver identify special CPU page table
entries.
-Locking with the update() callback is the most important aspect the driver must
-respect in order to keep things properly synchronized. The usage pattern is::
+Locking within the sync_cpu_device_pagetables() callback is the most important
+aspect the driver must respect in order to keep things properly synchronized.
+The usage pattern is::
int driver_populate_range(...)
{
struct hmm_range range;
...
+
+ range.notifier = &interval_sub;
+ range.start = ...;
+ range.end = ...;
+ range.hmm_pfns = ...;
+
+ if (!mmget_not_zero(interval_sub->notifier.mm))
+ return -EFAULT;
+
again:
- ret = hmm_vma_get_pfns(vma, &range, start, end, pfns);
- if (ret)
+ range.notifier_seq = mmu_interval_read_begin(&interval_sub);
+ mmap_read_lock(mm);
+ ret = hmm_range_fault(&range);
+ if (ret) {
+ mmap_read_unlock(mm);
+ if (ret == -EBUSY)
+ goto again;
return ret;
+ }
+ mmap_read_unlock(mm);
+
take_lock(driver->update);
- if (!hmm_vma_range_done(vma, &range)) {
+ if (mmu_interval_read_retry(&ni, range.notifier_seq) {
release_lock(driver->update);
goto again;
}
- // Use pfns array content to update device page table
+ /* Use pfns array content to update device page table,
+ * under the update lock */
release_lock(driver->update);
return 0;
}
The driver->update lock is the same lock that the driver takes inside its
-update() callback. That lock must be held before hmm_vma_range_done() to avoid
-any race with a concurrent CPU page table update.
+invalidate() callback. That lock must be held before calling
+mmu_interval_read_retry() to avoid any race with a concurrent CPU page table
+update.
-HMM implements all this on top of the mmu_notifier API because we wanted a
-simpler API and also to be able to perform optimizations latter on like doing
-concurrent device updates in multi-devices scenario.
+Leverage default_flags and pfn_flags_mask
+=========================================
-HMM also serves as an impedance mismatch between how CPU page table updates
-are done (by CPU write to the page table and TLB flushes) and how devices
-update their own page table. Device updates are a multi-step process. First,
-appropriate commands are written to a buffer, then this buffer is scheduled for
-execution on the device. It is only once the device has executed commands in
-the buffer that the update is done. Creating and scheduling the update command
-buffer can happen concurrently for multiple devices. Waiting for each device to
-report commands as executed is serialized (there is no point in doing this
-concurrently).
+The hmm_range struct has 2 fields, default_flags and pfn_flags_mask, that specify
+fault or snapshot policy for the whole range instead of having to set them
+for each entry in the pfns array.
+
+For instance if the device driver wants pages for a range with at least read
+permission, it sets::
+
+ range->default_flags = HMM_PFN_REQ_FAULT;
+ range->pfn_flags_mask = 0;
+
+and calls hmm_range_fault() as described above. This will fill fault all pages
+in the range with at least read permission.
+
+Now let's say the driver wants to do the same except for one page in the range for
+which it wants to have write permission. Now driver set::
+
+ range->default_flags = HMM_PFN_REQ_FAULT;
+ range->pfn_flags_mask = HMM_PFN_REQ_WRITE;
+ range->pfns[index_of_write] = HMM_PFN_REQ_WRITE;
+
+With this, HMM will fault in all pages with at least read (i.e., valid) and for the
+address == range->start + (index_of_write << PAGE_SHIFT) it will fault with
+write permission i.e., if the CPU pte does not have write permission set then HMM
+will call handle_mm_fault().
+
+After hmm_range_fault completes the flag bits are set to the current state of
+the page tables, ie HMM_PFN_VALID | HMM_PFN_WRITE will be set if the page is
+writable.
Represent and manage device memory from core kernel point of view
=================================================================
-Several different designs were tried to support device memory. First one used
-a device specific data structure to keep information about migrated memory and
-HMM hooked itself in various places of mm code to handle any access to
+Several different designs were tried to support device memory. The first one
+used a device specific data structure to keep information about migrated memory
+and HMM hooked itself in various places of mm code to handle any access to
addresses that were backed by device memory. It turns out that this ended up
replicating most of the fields of struct page and also needed many kernel code
paths to be updated to understand this new kind of memory.
@@ -271,97 +268,149 @@
unaware of the difference. We only need to make sure that no one ever tries to
map those pages from the CPU side.
-HMM provides a set of helpers to register and hotplug device memory as a new
-region needing a struct page. This is offered through a very simple API::
-
- struct hmm_devmem *hmm_devmem_add(const struct hmm_devmem_ops *ops,
- struct device *device,
- unsigned long size);
- void hmm_devmem_remove(struct hmm_devmem *devmem);
-
-The hmm_devmem_ops is where most of the important things are::
-
- struct hmm_devmem_ops {
- void (*free)(struct hmm_devmem *devmem, struct page *page);
- int (*fault)(struct hmm_devmem *devmem,
- struct vm_area_struct *vma,
- unsigned long addr,
- struct page *page,
- unsigned flags,
- pmd_t *pmdp);
- };
-
-The first callback (free()) happens when the last reference on a device page is
-dropped. This means the device page is now free and no longer used by anyone.
-The second callback happens whenever the CPU tries to access a device page
-which it cannot do. This second callback must trigger a migration back to
-system memory.
-
-
Migration to and from device memory
===================================
-Because the CPU cannot access device memory, migration must use the device DMA
-engine to perform copy from and to device memory. For this we need a new
-migration helper::
+Because the CPU cannot access device memory directly, the device driver must
+use hardware DMA or device specific load/store instructions to migrate data.
+The migrate_vma_setup(), migrate_vma_pages(), and migrate_vma_finalize()
+functions are designed to make drivers easier to write and to centralize common
+code across drivers.
- int migrate_vma(const struct migrate_vma_ops *ops,
- struct vm_area_struct *vma,
- unsigned long mentries,
- unsigned long start,
- unsigned long end,
- unsigned long *src,
- unsigned long *dst,
- void *private);
+Before migrating pages to device private memory, special device private
+``struct page`` need to be created. These will be used as special "swap"
+page table entries so that a CPU process will fault if it tries to access
+a page that has been migrated to device private memory.
-Unlike other migration functions it works on a range of virtual address, there
-are two reasons for that. First, device DMA copy has a high setup overhead cost
-and thus batching multiple pages is needed as otherwise the migration overhead
-makes the whole exercise pointless. The second reason is because the
-migration might be for a range of addresses the device is actively accessing.
+These can be allocated and freed with::
-The migrate_vma_ops struct defines two callbacks. First one (alloc_and_copy())
-controls destination memory allocation and copy operation. Second one is there
-to allow the device driver to perform cleanup operations after migration::
+ struct resource *res;
+ struct dev_pagemap pagemap;
- struct migrate_vma_ops {
- void (*alloc_and_copy)(struct vm_area_struct *vma,
- const unsigned long *src,
- unsigned long *dst,
- unsigned long start,
- unsigned long end,
- void *private);
- void (*finalize_and_map)(struct vm_area_struct *vma,
- const unsigned long *src,
- const unsigned long *dst,
- unsigned long start,
- unsigned long end,
- void *private);
- };
+ res = request_free_mem_region(&iomem_resource, /* number of bytes */,
+ "name of driver resource");
+ pagemap.type = MEMORY_DEVICE_PRIVATE;
+ pagemap.range.start = res->start;
+ pagemap.range.end = res->end;
+ pagemap.nr_range = 1;
+ pagemap.ops = &device_devmem_ops;
+ memremap_pages(&pagemap, numa_node_id());
-It is important to stress that these migration helpers allow for holes in the
-virtual address range. Some pages in the range might not be migrated for all
-the usual reasons (page is pinned, page is locked, ...). This helper does not
-fail but just skips over those pages.
+ memunmap_pages(&pagemap);
+ release_mem_region(pagemap.range.start, range_len(&pagemap.range));
-The alloc_and_copy() might decide to not migrate all pages in the
-range (for reasons under the callback control). For those, the callback just
-has to leave the corresponding dst entry empty.
+There are also devm_request_free_mem_region(), devm_memremap_pages(),
+devm_memunmap_pages(), and devm_release_mem_region() when the resources can
+be tied to a ``struct device``.
-Finally, the migration of the struct page might fail (for file backed page) for
-various reasons (failure to freeze reference, or update page cache, ...). If
-that happens, then the finalize_and_map() can catch any pages that were not
-migrated. Note those pages were still copied to a new page and thus we wasted
-bandwidth but this is considered as a rare event and a price that we are
-willing to pay to keep all the code simpler.
+The overall migration steps are similar to migrating NUMA pages within system
+memory (see :ref:`Page migration <page_migration>`) but the steps are split
+between device driver specific code and shared common code:
+1. ``mmap_read_lock()``
+
+ The device driver has to pass a ``struct vm_area_struct`` to
+ migrate_vma_setup() so the mmap_read_lock() or mmap_write_lock() needs to
+ be held for the duration of the migration.
+
+2. ``migrate_vma_setup(struct migrate_vma *args)``
+
+ The device driver initializes the ``struct migrate_vma`` fields and passes
+ the pointer to migrate_vma_setup(). The ``args->flags`` field is used to
+ filter which source pages should be migrated. For example, setting
+ ``MIGRATE_VMA_SELECT_SYSTEM`` will only migrate system memory and
+ ``MIGRATE_VMA_SELECT_DEVICE_PRIVATE`` will only migrate pages residing in
+ device private memory. If the latter flag is set, the ``args->pgmap_owner``
+ field is used to identify device private pages owned by the driver. This
+ avoids trying to migrate device private pages residing in other devices.
+ Currently only anonymous private VMA ranges can be migrated to or from
+ system memory and device private memory.
+
+ One of the first steps migrate_vma_setup() does is to invalidate other
+ device's MMUs with the ``mmu_notifier_invalidate_range_start(()`` and
+ ``mmu_notifier_invalidate_range_end()`` calls around the page table
+ walks to fill in the ``args->src`` array with PFNs to be migrated.
+ The ``invalidate_range_start()`` callback is passed a
+ ``struct mmu_notifier_range`` with the ``event`` field set to
+ ``MMU_NOTIFY_MIGRATE`` and the ``migrate_pgmap_owner`` field set to
+ the ``args->pgmap_owner`` field passed to migrate_vma_setup(). This is
+ allows the device driver to skip the invalidation callback and only
+ invalidate device private MMU mappings that are actually migrating.
+ This is explained more in the next section.
+
+ While walking the page tables, a ``pte_none()`` or ``is_zero_pfn()``
+ entry results in a valid "zero" PFN stored in the ``args->src`` array.
+ This lets the driver allocate device private memory and clear it instead
+ of copying a page of zeros. Valid PTE entries to system memory or
+ device private struct pages will be locked with ``lock_page()``, isolated
+ from the LRU (if system memory since device private pages are not on
+ the LRU), unmapped from the process, and a special migration PTE is
+ inserted in place of the original PTE.
+ migrate_vma_setup() also clears the ``args->dst`` array.
+
+3. The device driver allocates destination pages and copies source pages to
+ destination pages.
+
+ The driver checks each ``src`` entry to see if the ``MIGRATE_PFN_MIGRATE``
+ bit is set and skips entries that are not migrating. The device driver
+ can also choose to skip migrating a page by not filling in the ``dst``
+ array for that page.
+
+ The driver then allocates either a device private struct page or a
+ system memory page, locks the page with ``lock_page()``, and fills in the
+ ``dst`` array entry with::
+
+ dst[i] = migrate_pfn(page_to_pfn(dpage)) | MIGRATE_PFN_LOCKED;
+
+ Now that the driver knows that this page is being migrated, it can
+ invalidate device private MMU mappings and copy device private memory
+ to system memory or another device private page. The core Linux kernel
+ handles CPU page table invalidations so the device driver only has to
+ invalidate its own MMU mappings.
+
+ The driver can use ``migrate_pfn_to_page(src[i])`` to get the
+ ``struct page`` of the source and either copy the source page to the
+ destination or clear the destination device private memory if the pointer
+ is ``NULL`` meaning the source page was not populated in system memory.
+
+4. ``migrate_vma_pages()``
+
+ This step is where the migration is actually "committed".
+
+ If the source page was a ``pte_none()`` or ``is_zero_pfn()`` page, this
+ is where the newly allocated page is inserted into the CPU's page table.
+ This can fail if a CPU thread faults on the same page. However, the page
+ table is locked and only one of the new pages will be inserted.
+ The device driver will see that the ``MIGRATE_PFN_MIGRATE`` bit is cleared
+ if it loses the race.
+
+ If the source page was locked, isolated, etc. the source ``struct page``
+ information is now copied to destination ``struct page`` finalizing the
+ migration on the CPU side.
+
+5. Device driver updates device MMU page tables for pages still migrating,
+ rolling back pages not migrating.
+
+ If the ``src`` entry still has ``MIGRATE_PFN_MIGRATE`` bit set, the device
+ driver can update the device MMU and set the write enable bit if the
+ ``MIGRATE_PFN_WRITE`` bit is set.
+
+6. ``migrate_vma_finalize()``
+
+ This step replaces the special migration page table entry with the new
+ page's page table entry and releases the reference to the source and
+ destination ``struct page``.
+
+7. ``mmap_read_unlock()``
+
+ The lock can now be released.
Memory cgroup (memcg) and rss accounting
========================================
-For now device memory is accounted as any regular page in rss counters (either
+For now, device memory is accounted as any regular page in rss counters (either
anonymous if device page is used for anonymous, file if device page is used for
-file backed page or shmem if device page is used for shared memory). This is a
+file backed page, or shmem if device page is used for shared memory). This is a
deliberate choice to keep existing applications, that might start using device
memory without knowing about it, running unimpacted.
@@ -381,6 +430,6 @@
resource control.
-Note that device memory can never be pinned by device driver nor through GUP
+Note that device memory can never be pinned by a device driver nor through GUP
and thus such memory is always free upon process exit. Or when last reference
is dropped in case of shared memory or file backed memory.
--
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