linux-kernel - [RFC 6/6] mm, mempolicy: don't check cpuset seqlock where it doesn't matter

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Message-Id: <20170411140609.3787-7-vbabka@suse.cz>
Date:   Tue, 11 Apr 2017 16:06:09 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        Li Zefan <lizefan@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [RFC 6/6] mm, mempolicy: don't check cpuset seqlock where it doesn't matter

Two wrappers of __alloc_pages_nodemask() are checking task->mems_allowed_seq
themselves to retry allocation that has raced with a cpuset update. This has
been shown to be ineffective in preventing premature OOM's which can happen in
__alloc_pages_slowpath() long before it returns back to the wrappers to detect
the race at that level. Previous patches have made __alloc_pages_slowpath()
more robust, so we can now simply remove the seqlock checking in the wrappers
to prevent further wrong impression that it can actually help.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/mempolicy.c | 16 ----------------
 1 file changed, 16 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 72e5aeb1feeb..9a542b7a2189 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1900,12 +1900,9 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 	struct mempolicy *pol;
 	struct page *page;
 	int preferred_nid;
-	unsigned int cpuset_mems_cookie;
 	nodemask_t *nmask;
 
-retry_cpuset:
 	pol = get_vma_policy(vma, addr);
-	cpuset_mems_cookie = read_mems_allowed_begin();
 
 	if (pol->mode == MPOL_INTERLEAVE) {
 		unsigned nid;
@@ -1947,8 +1944,6 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 	page = __alloc_pages_nodemask(gfp, order, preferred_nid, nmask);
 	mpol_cond_put(pol);
 out:
-	if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie)))
-		goto retry_cpuset;
 	return page;
 }
 
@@ -1966,23 +1961,15 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
  *	Allocate a page from the kernel page pool.  When not in
  *	interrupt context and apply the current process NUMA policy.
  *	Returns NULL when no page can be allocated.
- *
- *	Don't call cpuset_update_task_memory_state() unless
- *	1) it's ok to take cpuset_sem (can WAIT), and
- *	2) allocating for current task (not interrupt).
  */
 struct page *alloc_pages_current(gfp_t gfp, unsigned order)
 {
 	struct mempolicy *pol = &default_policy;
 	struct page *page;
-	unsigned int cpuset_mems_cookie;
 
 	if (!in_interrupt() && !(gfp & __GFP_THISNODE))
 		pol = get_task_policy(current);
 
-retry_cpuset:
-	cpuset_mems_cookie = read_mems_allowed_begin();
-
 	/*
 	 * No reference counting needed for current->mempolicy
 	 * nor system default_policy
@@ -1994,9 +1981,6 @@ struct page *alloc_pages_current(gfp_t gfp, unsigned order)
 				policy_node(gfp, pol, numa_node_id()),
 				policy_nodemask(gfp, pol));
 
-	if (unlikely(!page && read_mems_allowed_retry(cpuset_mems_cookie)))
-		goto retry_cpuset;
-
 	return page;
 }
 EXPORT_SYMBOL(alloc_pages_current);
-- 
2.12.2