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    SubjectRe: [PATCH] zsmalloc documentation
    On 06/09/2012 04:29 AM, Konrad Rzeszutek Wilk wrote:

    > On Sun, Jun 03, 2012 at 01:20:05PM -0700, Nitin Gupta wrote:
    >> 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:
    >
    > I took your definition and grouped them a bit differently:
    > Not sure if it is more easier to read?
    >
    > page->first_page: points to the first component (0-order) page
    > 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.
    > page->lru: links together all component pages (except the first page)
    > of a zspage
    >
    > For _first_ page only:
    >
    > page->private (union with page->first_page): refers to the
    > component page after the first page
    > page->freelist: points to the first free object in zspage.
    > Free objects are linked together using in-place
    > metadata.
    > page->objects: maximum number of objects we can store in this
    > zspage (class->zspage_order * PAGE_SIZE / class->size)
    > page->lru: links together first pages of various zspages.
    > Basically forming list of zspages in a fullness group.
    > page->mapping: class index and fullness group of the zspage


    This regrouping indeed looks cleaner. I will send the revised patch soon.

    Thanks,
    Nitin


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