linux-kernel - Re: [PATCH 2/6] mm, tree wide: replace __GFP_REPEAT by __GFP_RETRY

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Date:   Mon, 26 Jun 2017 13:53:13 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Johannes Weiner <hannes@...xchg.org>, Mel Gorman <mgorman@...e.de>,
        NeilBrown <neilb@...e.com>, LKML <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 2/6] mm, tree wide: replace __GFP_REPEAT by
 __GFP_RETRY_MAYFAIL with more useful semantic

On 06/23/2017 10:53 AM, Michal Hocko wrote:
...

> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 4c6656f1fee7..6be1f836b69e 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -25,7 +25,7 @@ struct vm_area_struct;
>  #define ___GFP_FS		0x80u
>  #define ___GFP_COLD		0x100u
>  #define ___GFP_NOWARN		0x200u
> -#define ___GFP_REPEAT		0x400u
> +#define ___GFP_RETRY_MAYFAIL		0x400u
>  #define ___GFP_NOFAIL		0x800u
>  #define ___GFP_NORETRY		0x1000u
>  #define ___GFP_MEMALLOC		0x2000u
> @@ -136,26 +136,55 @@ struct vm_area_struct;
>   *
>   * __GFP_RECLAIM is shorthand to allow/forbid both direct and kswapd reclaim.
>   *
> - * __GFP_REPEAT: Try hard to allocate the memory, but the allocation attempt
> - *   _might_ fail.  This depends upon the particular VM implementation.
> + * The default allocator behavior depends on the request size. We have a concept
> + * of so called costly allocations (with order > PAGE_ALLOC_COSTLY_ORDER).
> + * !costly allocations are too essential to fail so they are implicitly
> + * non-failing (with some exceptions like OOM victims might fail) by default while
> + * costly requests try to be not disruptive and back off even without invoking
> + * the OOM killer. The following three modifiers might be used to override some of
> + * these implicit rules
> + *
> + * __GFP_NORETRY: The VM implementation will try only very lightweight
> + *   memory direct reclaim to get some memory under memory pressure (thus
> + *   it can sleep). It will avoid disruptive actions like OOM killer. The
> + *   caller must handle the failure which is quite likely to happen under
> + *   heavy memory pressure. The flag is suitable when failure can easily be
> + *   handled at small cost, such as reduced throughput
> + *
> + * __GFP_RETRY_MAYFAIL: The VM implementation will retry memory reclaim
> + *   procedures that have previously failed if there is some indication
> + *   that progress has been made else where.  It can wait for other
> + *   tasks to attempt high level approaches to freeing memory such as
> + *   compaction (which removes fragmentation) and page-out.
> + *   There is still a definite limit to the number of retries, but it is
> + *   a larger limit than with __GFP_NORERY.

Also, __GFP_NORETRY ^ (for grep purposes).