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

[Vlastimil Babka]

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 <vba...@suse.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