.. _zsmalloc:
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========
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zsmalloc
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========
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This allocator is designed for use with zram. Thus, the allocator is
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supposed to work well under low memory conditions. In particular, it
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never attempts higher order page allocation which is very likely to
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fail under memory pressure. On the other hand, if we just use single
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(0-order) pages, it would suffer from very high fragmentation --
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any object of size PAGE_SIZE/2 or larger would occupy an entire page.
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This was one of the major issues with its predecessor (xvmalloc).
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To overcome these issues, zsmalloc allocates a bunch of 0-order pages
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and links them together using various 'struct page' fields. These linked
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pages act as a single higher-order page i.e. an object can span 0-order
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page boundaries. The code refers to these linked pages as a single entity
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called zspage.
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For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
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since this satisfies the requirements of all its current users (in the
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worst case, page is incompressible and is thus stored "as-is" i.e. in
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uncompressed form). For allocation requests larger than this size, failure
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is returned (see zs_malloc).
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Additionally, zs_malloc() does not return a dereferenceable pointer.
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Instead, it returns an opaque handle (unsigned long) which encodes actual
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location of the allocated object. The reason for this indirection is that
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zsmalloc does not keep zspages permanently mapped since that would cause
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issues on 32-bit systems where the VA region for kernel space mappings
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is very small. So, before using the allocating memory, the object has to
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be mapped using zs_map_object() to get a usable pointer and subsequently
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unmapped using zs_unmap_object().
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stat
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====
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With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
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``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output::
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# cat /sys/kernel/debug/zsmalloc/zram0/classes
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class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage
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...
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...
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9 176 0 1 186 129 8 4
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10 192 1 0 2880 2872 135 3
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11 208 0 1 819 795 42 2
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12 224 0 1 219 159 12 4
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...
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...
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class
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index
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size
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object size zspage stores
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almost_empty
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the number of ZS_ALMOST_EMPTY zspages(see below)
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almost_full
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the number of ZS_ALMOST_FULL zspages(see below)
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obj_allocated
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the number of objects allocated
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obj_used
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the number of objects allocated to the user
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pages_used
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the number of pages allocated for the class
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pages_per_zspage
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the number of 0-order pages to make a zspage
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We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where
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* n = number of allocated objects
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* N = total number of objects zspage can store
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* f = fullness_threshold_frac(ie, 4 at the moment)
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Similarly, we assign zspage to:
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* ZS_ALMOST_FULL when n > N / f
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* ZS_EMPTY when n == 0
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* ZS_FULL when n == N
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