linux-kernel - Re: [PATCH 6/6] mm: discard __GFP

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Date:   Fri, 6 Jan 2023 09:35:24 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, NeilBrown <neilb@...e.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] mm: discard __GFP_ATOMIC

On Thu, Jan 05, 2023 at 03:49:44PM +0200, Mike Rapoport wrote:
> Hi Mel,
> 
> On Tue, Nov 29, 2022 at 03:17:01PM +0000, Mel Gorman wrote:
> > From: NeilBrown <neilb@...e.de>
> > 
> > __GFP_ATOMIC serves little purpose.  Its main effect is to set
> > ALLOC_HARDER which adds a few little boosts to increase the chance of an
> > allocation succeeding, one of which is to lower the water-mark at which it
> > will succeed.
> > 
> > It is *always* paired with __GFP_HIGH which sets ALLOC_HIGH which also
> > adjusts this watermark.  It is probable that other users of __GFP_HIGH
> > should benefit from the other little bonuses that __GFP_ATOMIC gets.
> > 
> > __GFP_ATOMIC also gives a warning if used with __GFP_DIRECT_RECLAIM.
> > There is little point to this.  We already get a might_sleep() warning if
> > __GFP_DIRECT_RECLAIM is set.
> > 
> > __GFP_ATOMIC allows the "watermark_boost" to be side-stepped.  It is
> > probable that testing ALLOC_HARDER is a better fit here.
> > 
> > __GFP_ATOMIC is used by tegra-smmu.c to check if the allocation might
> > sleep.  This should test __GFP_DIRECT_RECLAIM instead.
> > 
> > This patch:
> >  - removes __GFP_ATOMIC
> >  - allows __GFP_HIGH allocations to ignore watermark boosting as well
> >    as GFP_ATOMIC requests.
> >  - makes other adjustments as suggested by the above.
> > 
> > The net result is not change to GFP_ATOMIC allocations.  Other
> > allocations that use __GFP_HIGH will benefit from a few different extra
> > privileges.  This affects:
> >   xen, dm, md, ntfs3
> >   the vermillion frame buffer
> >   hibernation
> >   ksm
> >   swap
> > all of which likely produce more benefit than cost if these selected
> > allocation are more likely to succeed quickly.
> > 
> > [mgorman: Minor adjustments to rework on top of a series]
> > Link: https://lkml.kernel.org/r/163712397076.13692.4727608274002939094@noble.neil.brown.name
> > Signed-off-by: NeilBrown <neilb@...e.de>
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> > ---
> >  Documentation/mm/balance.rst   |  2 +-
> 
> Documentation/core-api/memory-allocation.rst needs an update as well, and
> there are other mentions of GFP_ATOMIC in Documentation/
> 

What part do you think needs updating in that file?

There are two references to GFP_ATOMIC in that file, one about accessing
reserves and another about non-sleeping allocations and the accuracy
does not change after the series. If anything, this statement should
change because it invites GFP_ATOMIC abuse for spurious reasons

  * If you think that accessing memory reserves is justified and the kernel
    will be stressed unless allocation succeeds, you may use ``GFP_ATOMIC``.

There are other references to GFP_ATOMIC in Documentation/ that are are a
little inaccurate but not in a way that is harmful and is not changed by
the series. For example;

	GFP_ATOMIC requests are kernel internal allocations that must
	be satisfied, immediately.  The kernel may drop some request,
	in rare cases even panic, if a GFP_ATOMIC alloc fails.

This is a stronger statement than GFP_ATOMIC deserves but it's close enough
in the given context.

-- 
Mel Gorman
SUSE Labs