Re: [PATCH v2 6/6] mm, mempolicy: don't check cpuset seqlock where it doesn't matter

Michal Hocko
2017-05-23T07:16:40Z

On Wed 17-05-17 10:11:40, Vlastimil Babka wrote:
> 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@suse.cz>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> mm/mempolicy.c | 16 ----------------
> 1 file changed, 16 deletions(-)
>
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 047181452040..7d8e56214ac0 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1898,12 +1898,9 @@ 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;
> @@ -1945,8 +1942,6 @@ 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;
> }
>
> @@ -1964,23 +1959,15 @@ 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
> @@ -1992,9 +1979,6 @@ 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

--
Michal Hocko
SUSE Labs