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mm: thp: optimize compound_trans_huge

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Currently we don't clobber page_tail->first_page during split_huge_page,
so compound_trans_head can be set to compound_head without adverse
effects, and this mostly optimizes away a smp_rmb.

It looks worthwhile to keep around the implementation that doesn't relay
on page_tail->first_page not to be clobbered, because it would be
necessary if we'll decide to enforce page->private to zero at all times
whenever PG_private is not set, also for anonymous pages.  For anonymous
pages enforcing such an invariant doesn't matter as anonymous pages
don't use page->private so we can get away with this microoptimization.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Pravin Shelar <pshelar@nicira.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andrea Arcangeli 2014-01-21 15:48:52 -08:00 committed by Linus Torvalds
parent ebf360f9bb
commit ca641514f4
1 changed files with 23 additions and 0 deletions
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23
include/linux/huge_mm.h
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@ -157,6 +157,26 @@ static inline int hpage_nr_pages(struct page *page)
return HPAGE_PMD_NR;
return 1;
}
/*
* compound_trans_head() should be used instead of compound_head(),
* whenever the "page" passed as parameter could be the tail of a
* transparent hugepage that could be undergoing a
* __split_huge_page_refcount(). The page structure layout often
* changes across releases and it makes extensive use of unions. So if
* the page structure layout will change in a way that
* page->first_page gets clobbered by __split_huge_page_refcount, the
* implementation making use of smp_rmb() will be required.
*
* Currently we define compound_trans_head as compound_head, because
* page->private is in the same union with page->first_page, and
* page->private isn't clobbered. However this also means we're
* currently leaving dirt into the page->private field of anonymous
* pages resulting from a THP split, instead of setting page->private
* to zero like for every other page that has PG_private not set. But
* anonymous pages don't use page->private so this is not a problem.
*/
#if 0
/* This will be needed if page->private will be clobbered in split_huge_page */
static inline struct page *compound_trans_head(struct page *page)
{
if (PageTail(page)) {
@ -174,6 +194,9 @@ static inline struct page *compound_trans_head(struct page *page)
}
return page;
}
#else
#define compound_trans_head(page) compound_head(page)
#endif
extern int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long addr, pmd_t pmd, pmd_t *pmdp);