Commit 8b164568 authored Oct 09, 2014 by Vlastimil Babka Committed by Linus Torvalds Oct 09, 2014

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It also deletes an outdated comment that assumed vma must be stable, as it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
9f1b868a ("mm: thp: khugepaged: add policy for finding target node").

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Minchan Kim <minchan@kernel.org>
Acked-by: Mel Gorman <mgorman@suse.de>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Nazarewicz <mina86@mina86.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 447f05bb

mm/huge_memory.c

+7 −13

Original line number	Diff line number	Diff line
		@@ -2322,23 +2322,17 @@ 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);

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