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Date:   Thu, 12 Jan 2023 09:43:41 +0000
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Michal Hocko <mhocko@suse.com>
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 5/7] mm/page_alloc.c: Allow __GFP_NOFAIL requests deeper
 access to reserves
Message-ID: <20230112094341.hom3ccscbko6v626@techsingularity.net>
References: <20230109151631.24923-1-mgorman@techsingularity.net>
 <20230109151631.24923-6-mgorman@techsingularity.net>
 <Y77ZxWRYe+4RPGRj@dhcp22.suse.cz>
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On Wed, Jan 11, 2023 at 04:46:13PM +0100, Michal Hocko wrote:
> On Mon 09-01-23 15:16:29, Mel Gorman wrote:
> > Currently __GFP_NOFAIL allocations without any other flags can access 25%
> > of the reserves but these requests imply that the system cannot make forward
> > progress until the allocation succeeds. Allow __GFP_NOFAIL access to 75%
> > of the min reserve.
> 
> I am not sure this is really needed. IIRC the original motivation for
> allowing NOFAIL request to access access to memory reserves was
> GFP_NOFS|__GFP_NOFAIL requests which do not invoke the OOM killer.
> The amount of memory reserves granted was not really important. The
> point was to allow to move forward. Giving more of the reserves is a
> double edge sword. It can help in some cases but it can also prevent
> other high priority users from fwd progress.
> 
> I would much rahter see such a change with an example where it really
> made a difference.
> 

Fair point but based on your review for "mm/page_alloc: Give GFP_ATOMIC
and non-blocking allocations access to reserves" and only allowing
non-blocking allocations to access reserves if __GFP_HIGH is also
specified, this patch becomes a no-op and can be dropped.

If GFP_NOFAIL requests really require deeper access to reserves, it'll
have to be explicitly handled in __zone_watermark_ok and __GFP_NOFAIL
would be added to the ALLOC_RESERVES collection of flags.

-- 
Mel Gorman
SUSE Labs