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    Subject[PATCH] zsmalloc documentation
    Date
    Signed-off-by: Nitin Gupta <ngupta@vflare.org>
    ---
    drivers/staging/zsmalloc/zsmalloc-main.c | 49 ++++++++++++++++++++++++++++++
    1 file changed, 49 insertions(+)

    diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
    index 4496737..f80f2fd 100644
    --- a/drivers/staging/zsmalloc/zsmalloc-main.c
    +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
    @@ -10,6 +10,55 @@
    * Released under the terms of GNU General Public License Version 2.0
    */

    +
    +/*
    + * This allocator is designed for use with zcache and zram. Thus, the
    + * allocator is supposed to work well under low memory conditions. In
    + * particular, it never attempts higher order page allocation which is
    + * very likely to fail under memory pressure. On the other hand, if we
    + * just use single (0-order) pages, it would suffer from very high
    + * fragmentation -- any object of size PAGE_SIZE/2 or larger would occupy
    + * an entire page. This was one of the major issues with its predecessor
    + * (xvmalloc).
    + *
    + * To overcome these issues, zsmalloc allocates a bunch of 0-order pages
    + * and links them together using various 'struct page' fields. These linked
    + * pages act as a single higher-order page i.e. an object can span 0-order
    + * page boundaries. The code refers to these linked pages as a single entity
    + * called zspage.
    + *
    + * Following is how we use various fields and flags of underlying
    + * struct page(s) to form a zspage.
    + *
    + * Usage of struct page fields:
    + * page->first_page: points to the first component (0-order) page
    + * [first] page->private (union with page->first_page): refers to the
    + * component page after the first page
    + *
    + * [first] page->freelist: points to the first free object in zspage.
    + * Free objects are linked together using in-place
    + * metadata.
    + * page->index (union with page->freelist): offset of the first object
    + * starting in this page. For the first page, this is
    + * always 0, so we use this field (aka freelist) to point
    + * to the first free object in zspage.
    + *
    + * [first] page->objects: maximum number of objects we can store in this
    + * zspage (class->zspage_order * PAGE_SIZE / class->size)
    + *
    + * page->lru: links together all component pages (except the first page)
    + * of a zspage
    + * [first] page->lru: links together first pages of various zspages.
    + * Basically forming list of zspages in a fullness group.
    + *
    + * [first] page->mapping: class index and fullness group of the zspage
    + *
    + * Usage of struct page flags:
    + * PG_private: identifies the first component page
    + * PG_private2: identifies the last component page
    + *
    + */
    +
    #ifdef CONFIG_ZSMALLOC_DEBUG
    #define DEBUG
    #endif
    --
    1.7.10.2


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