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b=g5VZa+oD7n0qwA8dkNw1dGt1hYXlL3zXO88zEqM7vtRhR1kV5deBd4b9OCOtJUWQ5 +N4pTKxXSl3IIDDDtWLcdaGHzf5YRCL5WIboBhYwWdxa9PoPaVWsgfnBXEDxHkH1eS xzx1SsHXbB59GXTRN3edLFEomnyLmfK6g4SSrm53cjWCgW8FYM8qzJ2gbI3L+AZzxV sjJLSgZNQmwcLieaLRLf5E+z1ii1Fdan/onsPk9Ph40+wV+r+20SjHboG/rT+ju6mz 11hHk1V/FKSJB5okOVcsvDJ52P9SN3ow8/R+JIR55Z7MMROrj45/EqEufFgUSEyF1E /Gne3r9EIYRGw== Message-ID: <294580c0-ad46-4160-b954-360a8a4dacdb@kernel.org> Date: Mon, 14 Sep 2026 10:16:33 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 09/26] mm/fbatch: restore mlock+munlock batching, without extra ref Content-Language: en-US To: Hugh Dickins Cc: Andrew Morton , Ackerley Tng , Alexander Viro , Alexandre Ghiti , Baolin Wang , Barry Song , Binbin Wu , Christian Brauner , Christoph Hellwig , Christoph Lameter , Claudio Imbrenda , David Hildenbrand , JP Kobryn , Jan Kara , Jens Axboe , Johannes Weiner , Kairui Song , Kiryl Shutsemau , Lance Yang , Leonardo Bras , Lorenzo Stoakes , Marcelo Tosatti , Matthew Wilcox , Mel Gorman , Miaohe Lin , Michal Hocko , Minchan Kim , Muchun Song , Oscar Salvador , Peter Zijlstra , Qi Zheng , Rik van Riel , Sebastian Andrzej Siewior , Shakeel Butt , Suren Baghdasaryan , Yang Shi , Yu Zhao , Zach O'Keefe , Zi Yan , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <69ba633c-8fd5-1f4b-2e1e-b9b15b8946be@google.com> <96fa2371-8a9e-8fcb-08f0-2977320bbbe8@google.com> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam04 X-Rspam-User: X-Stat-Signature: ux8e7hqtshjykbwhcc6sxajiougreia9 X-Rspamd-Queue-Id: 007E414000D X-HE-Tag: 1789373804-154043 X-HE-Meta: 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 L/LIv3pO ZCpr/tPBad6EX+E7X7uDtmD1iPLx67KXOeaEpnkhma9zjedZir92IrACCcKO5qVMWPaEsHOEEplYSbG4A+PWIBnqCfe4TeSeCRIBu4TMwxVSKZFvvgMXT+2ePbn5WECYDAOwKHAndCCZUDuP3pvRptti8Ac9fPS+rB13xpnLPOYUNUyv619pQar3t7Lzn4Yb25y5iSCccqOFDGn+sfiDGnWqoIDlsvP5l6CwpsCXyASxoABXNM3dAJ6HQHRyZLpma08w1iuA8/8Fmoo2b2qpuz9sbyzzFH0vCzQDno7gNy0QYdGZfmMU1AeusP6w8qq1dAdqk5m5y3UVyNI6aif12BN07GWPDrZAgT7n2LoLB2Ubs4Vk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/13/26 01:46, Hugh Dickins wrote: > On Thu, 10 Sep 2026, Vlastimil Babka (SUSE) wrote: >> On 9/9/26 11:59, Hugh Dickins wrote: >> > Update mlock_folio(), munlock_folio() and their fbatch callouts and >> > helpers, to do folio_try_get()s at batch processing time, instead of >> > holding a folio reference all the while in mlock_fbatch: as in folio.c. >> > >> > But more interesting is the use of mod_mlock_count(), using try_cmpxchg() >> > to update folio->mlock_count safely when possible (now when on lru_add >> > fbatch as well as when unevictable). While __mlock_folio() is as hard to >> > think about as before, __munlock_folio() simpler because munlock_folio() >> > can adjust mlock_count itself without clear_lru() or lruvec lock, and so >> > do the folio_test_clear_mlocked() immediately for itself (without which >> > unevictable_pgs_cleared was likely to appear high, when it should be 0 >> > or low to indicate good mlock health). >> > >> > __munlock_folio() is safe for use even when the unreferenced folio has >> > been freed and reused. It appears that __mlock_folio() could affect a >> > folio which has been freed and reused, but only if it is reused as an >> > mlocked folio, in which case its mlock_count is spuriously incremented >> > (but usually a spurious munlock decrement will follow). How grave is >> > this? If unevictable_pgs_cleared remains low, not so bad. >> > >> > I've gone back and forth on whether to move mlock_fbatch and these >> > functions into mm/folio.c: for now they stay here in mm/mlock.c. >> > >> > Signed-off-by: Hugh Dickins >> > --- >> > mm/mlock.c | 147 +++++++++++++++++++++++++++++++---------------------- >> > 1 file changed, 86 insertions(+), 61 deletions(-) >> > >> > diff --git a/mm/mlock.c b/mm/mlock.c >> > index 2c690f18031e..1050010bbe0b 100644 >> > --- a/mm/mlock.c >> > +++ b/mm/mlock.c >> > @@ -58,6 +58,20 @@ EXPORT_SYMBOL(can_do_mlock); >> > * indicate the unevictable state. >> > */ >> > >> > +static long mod_mlock_count(struct folio *folio, long incdec) >> > +{ >> > + long mlock_count = READ_ONCE(folio->mlock_count); >> > + >> > + while (mlock_count & MLOCK_COUNT_0) { >> > + if (mlock_count + incdec < MLOCK_COUNT_0) >> > + return MLOCK_COUNT_0; >> > + if (try_cmpxchg(&folio->mlock_count, &mlock_count, >> > + mlock_count + incdec)) >> > + return mlock_count + incdec; >> > + } >> >> This never rereads folio->mlock_count to mlock_count inside the loop, so it >> can spin forever? > > You give me a nasty moment, have I misunderstood? Isn't it part of the > try_cmpxchg() contract, that it reads folio->mlock_count into mlock_count > when it fails? Hence the "&mlock_count" rather than just "mlock_count"? Indeed, sorry! It would have helped if the contract was spelled out somewhere. But now I found it only for the atomic_try_cmpxchg() variant. Guess I'm too used to the slab_update_freelist() wrappers which don't follow this... but maybe should? Hmm. >> >> (too late here for me to understand the rest today) > > But that I understand very weil: thanks for all that you have managed. > > Hugh