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authorAndrea Arcangeli <aarcange@redhat.com>2014-01-21 15:48:52 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2014-01-21 16:19:43 -0800
commitca641514f4056deee1fb2eb356e2c99b98718ade (patch)
treec702724c845aa8510c9f658a18d66a3447f86ba4 /include/linux/huge_mm.h
parentebf360f9bb957f68e19e88f5067c015997dc26a6 (diff)
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mm: thp: optimize compound_trans_huge
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>
Diffstat (limited to 'include/linux/huge_mm.h')
-rw-r--r--include/linux/huge_mm.h23
1 files changed, 23 insertions, 0 deletions
diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
index 91672e2deec3..db512014e061 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);