On 06/09/2012 05:44 PM, Konrad Rzeszutek Wilk wrote: > On Sat, Jun 09, 2012 at 05:41:14PM -0700, Nitin Gupta wrote: >> Documentation of various struct page fields >> used by zsmalloc. >> >> Signed-off-by: Nitin Gupta <ngupta@xxxxxxxxxx> >> >> Changes for v2: >> - Regroup descriptions as suggested by Seth > ^^ - Konrad > > Otherwise: Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> > Sorry about that, Konrad! Greg: Please let me know if I should resend the patch with corrected name in the changelog. Thanks, Nitin >> --- >> drivers/staging/zsmalloc/zsmalloc-main.c | 48 ++++++++++++++++++++++++++++++ >> 1 file changed, 48 insertions(+) >> >> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c >> index 4496737..1db76ec 100644 >> --- a/drivers/staging/zsmalloc/zsmalloc-main.c >> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c >> @@ -10,6 +10,54 @@ >> * 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 >> + * 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 >> + * >> + * 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 >> >> -- >> To unsubscribe, send a message with 'unsubscribe linux-mm' in >> the body to majordomo@xxxxxxxxx. For more info on Linux MM, >> see: http://www.linux-mm.org/ . >> Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> >> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>