Messages in this thread Patch in this message | | | From | Andrea Arcangeli <> | Subject | [PATCH 4/8] mm: thp: optimize compound_trans_huge | Date | Wed, 20 Nov 2013 18:51:12 +0100 |
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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> --- include/linux/huge_mm.h | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h index 91672e2..db51201 100644 --- a/include/linux/huge_mm.h +++ b/include/linux/huge_mm.h @@ -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);
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