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

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Date:   Tue, 23 May 2017 09:16:40 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, Li Zefan <lizefan@...wei.com>,
        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>
Subject: Re: [PATCH v2 6/6] mm, mempolicy: don't check cpuset seqlock where
 it doesn't matter

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@...e.cz>

Acked-by: Michal Hocko <mhocko@...e.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