linux-kernel - Re: [PATCH] mm/gup: Drain batched mlock folio processing before attempting migration

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Message-ID: <24905f30-4a7c-e627-a272-80977204ed68@google.com>
Date: Fri, 29 Aug 2025 09:04:37 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Will Deacon <will@...nel.org>
cc: Hugh Dickins <hughd@...gle.com>, David Hildenbrand <david@...hat.com>, 
    linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
    Keir Fraser <keirf@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>, 
    John Hubbard <jhubbard@...dia.com>, Frederick Mayle <fmayle@...gle.com>, 
    Andrew Morton <akpm@...ux-foundation.org>, Peter Xu <peterx@...hat.com>, 
    Rik van Riel <riel@...riel.com>, Vlastimil Babka <vbabka@...e.cz>, 
    Ge Yang <yangge1116@....com>
Subject: Re: [PATCH] mm/gup: Drain batched mlock folio processing before
 attempting migration

On Fri, 29 Aug 2025, Will Deacon wrote:
> On Fri, Aug 29, 2025 at 12:57:43PM +0100, Will Deacon wrote:
> > On Thu, Aug 28, 2025 at 01:47:14AM -0700, Hugh Dickins wrote:
> > > Patch is against 6.17-rc3; but if you'd prefer the patch against 6.12
> > > (or an intervening release), I already did the backport so please just
> > > ask.
> > 
> > We've got 6.15 working well at the moment, so I'll backport your diff
> > to that.
> 
> Notwithstanding my question about the synchronisation, I cherry-picked
> 86ebd50224c0 ("mm: add folio_expected_ref_count() for reference count
> calculation") to my 6.15-based Android tree and applied your diff on top.

Yes, I cherry-picked exactly that into my 6.12.  Not a big deal,
but a word of advance warning: the first patch in my series will
(I believe) be a fix to that folio_expected_ref_count(), to allow
for PG_private_2, which implies +1 on refcount (I've not yet
researched whether it's +1 or +2 if PG_private and PG_private_2
are both set - comments I've seen imply +1 but I need to check).

I thought of that when doing the cherry-pick because I thought that
PG_private_2 had already been killed off by now in the latest tree:
but was surprised to find that it is still there in 6.17-rc.

> 
> With that, I've not managed to reproduce the original failure and
> haven't observed any migration failures on the GUP path.

Great, many thanks to you, Will.

Hugh