[PATCH V4 01/15] mm, THP: don't hold mmap_sem in khugepaged when allocating THP

[Vlastimil Babka]

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also deletes an outdated comment that assumed vma must be stable, as it was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 9f1b868a13
("mm: thp: khugepaged: add policy for finding target node").

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Cc: 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>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
---
 mm/huge_memory.c | 20 +++++++-------------
 1 file changed, 7 insertions(+), 13 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 02559ef..107da28 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2295,23 +2295,17 @@ 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);
-- 
1.8.4.5

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