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Date:   Thu, 12 Jan 2023 09:29:16 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Linux-MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        NeilBrown <neilb@suse.de>,
        Thierry Reding <thierry.reding@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 6/7] mm/page_alloc: Give GFP_ATOMIC and non-blocking
 allocations access to reserves
Message-ID: <Y7/E3PRYqgw1R1HI@dhcp22.suse.cz>
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On Thu 12-01-23 09:11:07, Michal Hocko wrote:
> On Wed 11-01-23 17:05:52, Mel Gorman wrote:
> > On Wed, Jan 11, 2023 at 04:58:02PM +0100, Michal Hocko wrote:
> > > On Mon 09-01-23 15:16:30, Mel Gorman wrote:
> > > > Explicit GFP_ATOMIC allocations get flagged ALLOC_HARDER which is a bit
> > > > vague. In preparation for removing __GFP_ATOMIC, give GFP_ATOMIC and
> > > > other non-blocking allocation requests equal access to reserve.  Rename
> > > > ALLOC_HARDER to ALLOC_NON_BLOCK to make it more clear what the flag
> > > > means.
> > > 
> > > GFP_NOWAIT can be also used for opportunistic allocations which can and
> > > should fail quickly if the memory is tight and more elaborate path
> > > should be taken (e.g. try higher order allocation first but fall back to
> > > smaller request if the memory is fragmented). Do we really want to give
> > > those access to memory reserves as well?
> > 
> > Good question. Without __GFP_ATOMIC, GFP_NOWAIT only differs from GFP_ATOMIC
> > by __GFP_HIGH but that is not enough to distinguish between a caller that
> > cannot sleep versus one that is speculatively attempting an allocation but
> > has other options. That changelog is misleading, it's not equal access
> > as GFP_NOWAIT ends up with 25% of the reserves which is less than what
> > GFP_ATOMIC gets.
> > 
> > Because it becomes impossible to distinguish between non-blocking and
> > atomic without __GFP_ATOMIC, there is some justification for allowing
> > access to reserves for GFP_NOWAIT. bio for example attempts an allocation
> > (clears __GFP_DIRECT_RECLAIM) before falling back to mempool but delays
> > in IO can also lead to further allocation pressure. mmu gather failing
> > GFP_WAIT slows the rate memory can be freed. NFS failing GFP_NOWAIT will
> > have to retry IOs multiple times. The examples were picked at random but
> > the point is that there are cases where failing GFP_NOWAIT can degrade
> > the system, particularly delay the cleaning of pages before reclaim.
> 
> Fair points.
> 
> > A lot of the truly speculative users appear to use GFP_NOWAIT | __GFP_NOWARN
> > so one compromise would be to avoid using reserves if __GFP_NOWARN is
> > also specified.
> > 
> > Something like this as a separate patch?
> 
> I cannot say I would be happy about adding more side effects to
> __GFP_NOWARN. You are right that it should be used for those optimistic
> allocation requests but historically all many of these subtle side effects
> have kicked back at some point.

Should have looked at git grep GFP_ATOMIC | __GFP_NOWARN is quite
popular with more than 50 instances.

-- 
Michal Hocko
SUSE Labs