linux-kernel - Re: [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

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Message-ID: <CAOUHufYyEjDG1+MEqRhuWPL037aSqrDhi_FT5gfyktbpQBmDVA@mail.gmail.com>
Date: Sun, 27 Oct 2024 18:24:54 -0600
From: Yu Zhao <yuzhao@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Zi Yan <ziy@...dia.com>, Mel Gorman <mgorman@...hsingularity.net>, 
	Matt Fleming <mfleming@...udflare.com>, David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Link Lin <linkl@...gle.com>
Subject: Re: [PATCH mm-unstable v2] mm/page_alloc: keep track of free highatomic

On Sun, Oct 27, 2024 at 3:05 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 10/27/24 21:51, Yu Zhao wrote:
> > On Sun, Oct 27, 2024 at 2:36 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >> On 10/27/24 21:17, Yu Zhao wrote:
> >> > On Sun, Oct 27, 2024 at 1:53 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> >> >>
> >>
> >> For example:
> >>
> >> - a page is on pcplist in MIGRATE_MOVABLE list
> >> - we reserve its pageblock as highatomic, which does nothing to the page on
> >> the pcplist
> >> - page above is flushed from pcplist to zone freelist, but it remembers it
> >> was MIGRATE_MOVABLE, merges with another buddy/buddies from the
> >> now-highatomic list, the resulting order-X page ends up on the movable
> >> freelist despite being in highatomic pageblock. The counter of free
> >> highatomic is now wrong wrt the freelist reality
> >
> > This is the part I don't follow: how is it wrong w.r.t. the freelist
> > reality? The new nr_free_highatomic should reflect how many pages are
> > exactly on free_list[MIGRATE_HIGHATOMIC], because it's updated
> > accordingly.
>
> You'd have to try implementing your change in the kernel without that
> migratetype hygiene series, and see how it would either not work, or you'd
> end up implementing the series as part of that.

A proper backport would need to track the MT of the free_list a page
is deleted from, and I see no reason why in such a proper backport
"the counter could drift easily" or "the counter of free highatomic is
now wrong wrt the freelist reality". So I assume you actually mean
this patch can't be backported cleanly? (Which I do agree.)