linux-kernel - Re: [PATCH 2/6] MM: improve documentation for __GFP

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Date:   Tue, 5 Oct 2021 11:20:51 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     NeilBrown <neilb@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.com>,
        ". Dave Chinner" <david@...morbit.com>,
        Jonathan Corbet <corbet@....net>
Cc:     linux-xfs@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH 2/6] MM: improve documentation for __GFP_NOFAIL

On 9/17/21 04:56, NeilBrown wrote:
> __GFP_NOFAIL is documented both in gfp.h and memory-allocation.rst.
> The details are not entirely consistent.
> 
> This patch ensures both places state that:
>  - there is a risk of deadlock with reclaim/writeback/oom-kill
>  - it should only be used when there is no real alternative
>  - it is preferable to an endless loop
>  - it is strongly discourages for costly-order allocations.
> 
> Signed-off-by: NeilBrown <neilb@...e.de>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

Nit below:

> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index 55b2ec1f965a..1d2a89e20b8b 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -209,7 +209,11 @@ struct vm_area_struct;
>   * used only when there is no reasonable failure policy) but it is
>   * definitely preferable to use the flag rather than opencode endless
>   * loop around allocator.
> - * Using this flag for costly allocations is _highly_ discouraged.
> + * Use of this flag may lead to deadlocks if locks are held which would
> + * be needed for memory reclaim, write-back, or the timely exit of a
> + * process killed by the OOM-killer.  Dropping any locks not absolutely
> + * needed is advisable before requesting a %__GFP_NOFAIL allocate.
> + * Using this flag for costly allocations (order>1) is _highly_ discouraged.

We define costly as 3, not 1. But sure it's best to avoid even order>0 for
__GFP_NOFAIL. Advising order>1 seems arbitrary though?

>   */
>  #define __GFP_IO	((__force gfp_t)___GFP_IO)
>  #define __GFP_FS	((__force gfp_t)___GFP_FS)
> 
> 
>