[RFC 1/6] mm, page_alloc: fix more premature OOM due to race with cpuset update

[Vlastimil Babka <vbabka@suse.cz>]

Commit e47483bca2cc ("mm, page_alloc: fix premature OOM when racing with cpuset
mems update") has fixed known recent regressions found by LTP's cpuset01
testcase. I have however found that by modifying the testcase to use per-vma
mempolicies via bind(2) instead of per-task mempolicies via set_mempolicy(2),
the premature OOM still happens and the issue is much older.

The root of the problem is that the cpuset's mems_allowed and mempolicy's
nodemask can temporarily have no intersection, thus get_page_from_freelist()
cannot find any usable zone. The current semantic for empty intersection is to
ignore mempolicy's nodemask and honour cpuset restrictions. This is checked in
node_zonelist(), but the racy update can happen after we already passed the
check. Such races should be protected by the seqlock task->mems_allowed_seq,
but it doesn't work here, because 1) mpol_rebind_mm() does not happen under
seqlock for write, and doing so would lead to deadlock, as it takes mmap_sem
for write, while the allocation can have mmap_sem for read when it's taking the
seqlock for read. And 2) the seqlock cookie of callers of node_zonelist()
(alloc_pages_vma() and alloc_pages_current()) is different than the one of
__alloc_pages_slowpath(), so there's still a potential race window.

This patch fixes the issue by having __alloc_pages_slowpath() check for empty
intersection of cpuset and ac->nodemask before OOM or allocation failure. If
it's indeed empty, the nodemask is ignored and allocation retried, which mimics
node_zonelist(). This works fine, because almost all callers of
__alloc_pages_nodemask are obtaining the nodemask via node_zonelist(). The only
exception is new_node_page() from hotplug, where the potential violation of
nodemask isn't an issue, as there's already a fallback allocation attempt
without any nodemask. If there's a future caller that needs to have its specific
nodemask honoured over task's cpuset restrictions, we'll have to e.g. add a gfp
flag for that.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/page_alloc.c | 51 ++++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 38 insertions(+), 13 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 32b31d661c9c..502d82f0e004 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3668,6 +3668,39 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
 	return false;
 }
 
+static inline bool
+check_retry_cpuset(int cpuset_mems_cookie, struct alloc_context *ac)
+{
+	/*
+	 * It's possible that cpuset's mems_allowed and the nodemask from
+	 * mempolicy don't intersect. This should be normally dealt with by
+	 * policy_nodemask(), but it's possible to race with cpuset update in
+	 * such a way the check therein was true, and then it became false
+	 * before we got our cpuset_mems_cookie here.
+	 * This assumes that for all allocations, ac->nodemask can come only
+	 * from MPOL_BIND mempolicy (whose documented semantics is to be ignored
+	 * when it does not intersect with the cpuset restrictions) or the
+	 * caller can deal with a violated nodemask.
+	 */
+	if (cpusets_enabled() && ac->nodemask &&
+			!cpuset_nodemask_valid_mems_allowed(ac->nodemask)) {
+		ac->nodemask = NULL;
+		return true;
+	}
+
+	/*
+	 * When updating a task's mems_allowed or mempolicy nodemask, it is
+	 * possible to race with parallel threads in such a way that our
+	 * allocation can fail while the mask is being updated. If we are about
+	 * to fail, check if the cpuset changed during allocation and if so,
+	 * retry.
+	 */
+	if (read_mems_allowed_retry(cpuset_mems_cookie))
+		return true;
+
+	return false;
+}
+
 static inline struct page *
 __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 						struct alloc_context *ac)
@@ -3863,11 +3896,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 				&compaction_retries))
 		goto retry;
 
-	/*
-	 * It's possible we raced with cpuset update so the OOM would be
-	 * premature (see below the nopage: label for full explanation).
-	 */
-	if (read_mems_allowed_retry(cpuset_mems_cookie))
+
+	/* Deal with possible cpuset update races before we start OOM killing */
+	if (check_retry_cpuset(cpuset_mems_cookie, ac))
 		goto retry_cpuset;
 
 	/* Reclaim has failed us, start killing things */
@@ -3886,14 +3917,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	}
 
 nopage:
-	/*
-	 * When updating a task's mems_allowed or mempolicy nodemask, it is
-	 * possible to race with parallel threads in such a way that our
-	 * allocation can fail while the mask is being updated. If we are about
-	 * to fail, check if the cpuset changed during allocation and if so,
-	 * retry.
-	 */
-	if (read_mems_allowed_retry(cpuset_mems_cookie))
+	/* Deal with possible cpuset update races before we fail */
+	if (check_retry_cpuset(cpuset_mems_cookie, ac))
 		goto retry_cpuset;
 
 	/*
-- 
2.12.2

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