linux-kernel - Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into alloc_hugepage_direct

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Date:   Wed, 26 Sep 2018 16:30:39 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Argangeli <andrea@...nel.org>,
        Zi Yan <zi.yan@...rutgers.edu>,
        Stefan Priebe - Profihost AG <s.priebe@...fihost.ag>,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/2] mm, thp: consolidate THP gfp handling into
 alloc_hugepage_direct_gfpmask

On Tue, Sep 25, 2018 at 02:03:26PM +0200, Michal Hocko wrote:
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c3bc7e9c9a2a..c0bcede31930 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -629,21 +629,40 @@ static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf,
>   *	    available
>   * never: never stall for any thp allocation
>   */
> -static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma)
> +static inline gfp_t alloc_hugepage_direct_gfpmask(struct vm_area_struct *vma, unsigned long addr)
>  {
>  	const bool vma_madvised = !!(vma->vm_flags & VM_HUGEPAGE);
> +	gfp_t this_node = 0;
> +
> +#ifdef CONFIG_NUMA
> +	struct mempolicy *pol;
> +	/*
> +	 * __GFP_THISNODE is used only when __GFP_DIRECT_RECLAIM is not
> +	 * specified, to express a general desire to stay on the current
> +	 * node for optimistic allocation attempts. If the defrag mode
> +	 * and/or madvise hint requires the direct reclaim then we prefer
> +	 * to fallback to other node rather than node reclaim because that
> +	 * can lead to excessive reclaim even though there is free memory
> +	 * on other nodes. We expect that NUMA preferences are specified
> +	 * by memory policies.
> +	 */
> +	pol = get_vma_policy(vma, addr);
> +	if (pol->mode != MPOL_BIND)
> +		this_node = __GFP_THISNODE;
> +	mpol_cond_put(pol);
> +#endif

I'm not very good with NUMA policies. Could you explain in more details how
the code above is equivalent to the code below?

...

> @@ -2026,60 +2025,6 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
>  		goto out;
>  	}
>  
> -	if (unlikely(IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && hugepage)) {
> -		int hpage_node = node;
> -
> -		/*
> -		 * For hugepage allocation and non-interleave policy which
> -		 * allows the current node (or other explicitly preferred
> -		 * node) we only try to allocate from the current/preferred
> -		 * node and don't fall back to other nodes, as the cost of
> -		 * remote accesses would likely offset THP benefits.
> -		 *
> -		 * If the policy is interleave, or does not allow the current
> -		 * node in its nodemask, we allocate the standard way.
> -		 */
> -		if (pol->mode == MPOL_PREFERRED &&
> -						!(pol->flags & MPOL_F_LOCAL))
> -			hpage_node = pol->v.preferred_node;
> -
> -		nmask = policy_nodemask(gfp, pol);
> -		if (!nmask || node_isset(hpage_node, *nmask)) {
> -			mpol_cond_put(pol);
> -			/*
> -			 * We cannot invoke reclaim if __GFP_THISNODE
> -			 * is set. Invoking reclaim with
> -			 * __GFP_THISNODE set, would cause THP
> -			 * allocations to trigger heavy swapping
> -			 * despite there may be tons of free memory
> -			 * (including potentially plenty of THP
> -			 * already available in the buddy) on all the
> -			 * other NUMA nodes.
> -			 *
> -			 * At most we could invoke compaction when
> -			 * __GFP_THISNODE is set (but we would need to
> -			 * refrain from invoking reclaim even if
> -			 * compaction returned COMPACT_SKIPPED because
> -			 * there wasn't not enough memory to succeed
> -			 * compaction). For now just avoid
> -			 * __GFP_THISNODE instead of limiting the
> -			 * allocation path to a strict and single
> -			 * compaction invocation.
> -			 *
> -			 * Supposedly if direct reclaim was enabled by
> -			 * the caller, the app prefers THP regardless
> -			 * of the node it comes from so this would be
> -			 * more desiderable behavior than only
> -			 * providing THP originated from the local
> -			 * node in such case.
> -			 */
> -			if (!(gfp & __GFP_DIRECT_RECLAIM))
> -				gfp |= __GFP_THISNODE;
> -			page = __alloc_pages_node(hpage_node, gfp, order);
> -			goto out;
> -		}
> -	}
> -
>  	nmask = policy_nodemask(gfp, pol);
>  	preferred_nid = policy_node(gfp, pol, node);
>  	page = __alloc_pages_nodemask(gfp, order, preferred_nid, nmask);

-- 
 Kirill A. Shutemov