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

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Date:   Tue, 5 Oct 2021 14:27:45 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...e.com>
Cc:     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>,
        ". Dave Chinner" <david@...morbit.com>,
        Jonathan Corbet <corbet@....net>, 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 10/5/21 13:09, Michal Hocko wrote:
> On Tue 05-10-21 11:20:51, Vlastimil Babka wrote:
> [...]
>> > --- 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?
> 
> This is not completely arbitrary. We have a warning for any higher order
> allocation.
> rmqueue:
> 	WARN_ON_ONCE((gfp_flags & __GFP_NOFAIL) && (order > 1));

Oh, I missed that.

> I do agree that "Using this flag for higher order allocations is
> _highly_ discouraged.

Well, with the warning in place this is effectively forbidden, not just
discouraged.

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