The patch titled
Subject: mm, THP: don't hold mmap_sem in khugepaged when allocating THP
has been added to the -mm tree. Its filename is
mm-thp-dont-hold-mmap_sem-in-khugepaged-when-allocating-thp.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/mm-thp-dont-hold-mmap_sem-in-khugepaged-when-allocating-thp.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/mm-thp-dont-hold-mmap_sem-in-khugepaged-when-allocating-thp.patch
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From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm, THP: don't hold mmap_sem in khugepaged when allocating THP
When allocating huge page for collapsing, khugepaged currently holds
mmap_sem for reading on the mm where collapsing occurs. Afterwards the
read lock is dropped before write lock is taken on the same mmap_sem.
Holding mmap_sem during whole huge page allocation is therefore useless,
the vma needs to be rechecked after taking the write lock anyway.
Furthemore, huge page allocation might involve a rather long sync
compaction, and thus block any mmap_sem writers and i.e. affect workloads
that perform frequent m(un)map or mprotect oterations.
This patch simply releases the read lock before allocating a huge page.
It also deletes an outdated comment that assumed vma must be stable, as it
was using alloc_hugepage_vma(). This is no longer true since commit
9f1b868a13ac ("mm: thp: khugepaged: add policy for finding target node").
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/huge_memory.c | 20 +++++++-------------
1 file changed, 7 insertions(+), 13 deletions(-)
diff -puN mm/huge_memory.c~mm-thp-dont-hold-mmap_sem-in-khugepaged-when-allocating-thp mm/huge_memory.c
--- a/mm/huge_memory.c~mm-thp-dont-hold-mmap_sem-in-khugepaged-when-allocating-thp
+++ a/mm/huge_memory.c
@@ -2319,23 +2319,17 @@ static struct page
int node)
{
VM_BUG_ON_PAGE(*hpage, *hpage);
+
/*
- * Allocate the page while the vma is still valid and under
- * the mmap_sem read mode so there is no memory allocation
- * later when we take the mmap_sem in write mode. This is more
- * friendly behavior (OTOH it may actually hide bugs) to
- * filesystems in userland with daemons allocating memory in
- * the userland I/O paths. Allocating memory with the
- * mmap_sem in read mode is good idea also to allow greater
- * scalability.
+ * Before allocating the hugepage, release the mmap_sem read lock.
+ * The allocation can take potentially a long time if it involves
+ * sync compaction, and we do not need to hold the mmap_sem during
+ * that. We will recheck the vma after taking it again in write mode.
*/
+ up_read(&mm->mmap_sem);
+
*hpage = alloc_pages_exact_node(node, alloc_hugepage_gfpmask(
khugepaged_defrag(), __GFP_OTHER_NODE), HPAGE_PMD_ORDER);
- /*
- * After allocating the hugepage, release the mmap_sem read lock in
- * preparation for taking it in write mode.
- */
- up_read(&mm->mmap_sem);
if (unlikely(!*hpage)) {
count_vm_event(THP_COLLAPSE_ALLOC_FAILED);
*hpage = ERR_PTR(-ENOMEM);
_
Patches currently in -mm which might be from vbabka@xxxxxxx are
mm-page_alloc-determine-migratetype-only-once.patch
mm-thp-dont-hold-mmap_sem-in-khugepaged-when-allocating-thp.patch
mm-compaction-defer-each-zone-individually-instead-of-preferred-zone.patch
mm-compaction-do-not-count-compact_stall-if-all-zones-skipped-compaction.patch
mm-compaction-do-not-recheck-suitable_migration_target-under-lock.patch
mm-compaction-move-pageblock-checks-up-from-isolate_migratepages_range.patch
mm-compaction-reduce-zone-checking-frequency-in-the-migration-scanner.patch
mm-compaction-khugepaged-should-not-give-up-due-to-need_resched.patch
mm-compaction-periodically-drop-lock-and-restore-irqs-in-scanners.patch
mm-compaction-skip-rechecks-when-lock-was-already-held.patch
mm-compaction-remember-position-within-pageblock-in-free-pages-scanner.patch
mm-compaction-skip-buddy-pages-by-their-order-in-the-migrate-scanner.patch
mm-rename-allocflags_to_migratetype-for-clarity.patch
mm-compaction-pass-gfp-mask-to-compact_control.patch
mm-compactionc-isolate_freepages_block-small-tuneup.patch
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