linux-kernel - Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when demoting pages

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Message-ID: <ZyDft_sCKm2vBF1j@PC2K9PVX.TheFacebook.com>
Date: Tue, 29 Oct 2024 09:14:31 -0400
From: Gregory Price <gourry@...rry.net>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: Yang Shi <shy828301@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, kernel-team@...a.com, akpm@...ux-foundation.org,
	weixugc@...gle.com, dave.hansen@...ux.intel.com, osalvador@...e.de,
	stable@...r.kernel.org
Subject: Re: [PATCH] vmscan,migrate: fix double-decrement on node stats when
 demoting pages

On Tue, Oct 29, 2024 at 08:34:34AM +0800, Huang, Ying wrote:
> Gregory Price <gourry@...rry.net> writes:
> 
> > On Mon, Oct 28, 2024 at 01:45:48PM -0700, Yang Shi wrote:
> >> On Fri, Oct 25, 2024 at 7:17 AM Gregory Price <gourry@...rry.net> wrote:
> >> >
> >> > This path happens for SUCCESSFUL migrations, not failures. Typically
> >> > callers to migrate_pages are required to handle putback/accounting for
> >> > failures, but this is already handled in the shrink code.
> >> 
> >> AFAIK, MGLRU doesn't dec/inc this counter, so it is not
> >> double-decrement for MGLRU. Maybe "imbalance update" is better?
> >> Anyway, it is just a nit. I'd suggest capturing the MGLRU case in the
> >> commit log too.
> >>
> >
> > Gotcha, so yeah saying it's an imbalance fix is more accurate.
> >
> > So more accurate changelog is:
... 
> 
> I think that it may be better to mention the different behavior of LRU
> and MGLRU.  But that's not a big deal, change it again only if you think
> it's necessary.
>

The behavior isn't really different. It's either way migrate_pages decrements
when it shouldn't going through the shink code - and both LRU and MGLRU go
through the same code.  That LRU does an inc/dec pair is irrelevant - neither
should do the decrement in the migrate path.

~Gregory