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b=xe4wnLPMzAF02PpoBtQdWpAESwFpnQgwqHUHHMwArKEQ3kVC7isdYXV2pWRF8ZeQW D8+bH/HFkmCtEHk2qeU9BcJuDhoUKKswmdshZhvxE/RT/Qinu2ONkvRa+8SO0iCfKZ LhOCPmhz4FG8PZFU9nRsJ6OBYvPcr+vFVAaHf4H4= Date: Tue, 15 Sep 2026 18:46:33 -0700 From: Andrew Morton To: Gregory Price Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, liam@infradead.org, ljs@kernel.org, david@kernel.org, vbabka@kernel.org, jannh@google.com, sashiko-bot , stable@vger.kernel.org Subject: Re: [RESEND PATCH] mm/madvise: use folio_trylock() in the cold/pageout PMD split Message-Id: <20260915184633.e2d8cf719e5eed186d087a8a@linux-foundation.org> In-Reply-To: <20260912110540.3203010-1-gourry@gourry.net> References: <20260912110540.3203010-1-gourry@gourry.net> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: byzkp76ftncb4qguue41fus74wojay4j X-Rspamd-Queue-Id: BFC8E40002 X-Rspamd-Server: rspam07 X-HE-Tag: 1789523195-200360 X-HE-Meta: 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 kykTjq6Q p5Ywtl1nso8h15XbFflbhmqZIzlNsrN3RMX2+vZgROwqxxF2fROByWxXDfrinFTPqxomzZPkKUCwMRqSYnTscx3oNmSsGDpX0KFG8gFlvlCDLUWOXGBaObed6z1HXX8bUAqPcb2Dg+yk9teFL5+sWBHfHxmyhDIzQAJq0z83cQhGm2SssMJI2C5M0C4Kyr52H6SLsAx9IuVwYeM3iNkwLxdLNlFvbFzNDEth/PyD/iIIPVyzaQjDM1ta5lm4U16n5YKoA24rYFRP/mpMIwqmJsAp6bwjrZNDOzK0fML3UoG8+RWNJD1f5SVQQv3vjBqAOuNPf+PaE8pHxwNQITlLWTL8q8dfMR2UeqU2nR/zpNtoadAOkBDObF54yyN5+Sgss15FlxB0foOon/sXeb4wN766TcrgY/hBTb8vt Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sat, 12 Sep 2026 07:05:40 -0400 Gregory Price wrote: > MADV_COLD or MADV_PAGEOUT over part of a PMD splits the THP in > madvise_cold_or_pageout_pte_range(). Two threads doing that to > the same THP create spurious failures. > > CPU0 CPU1 > ---- ---- > folio_get() > spin_unlock(ptl) > folio_lock() > folio_get() > spin_unlock(ptl) > folio_lock() <- blocks, keeps its ref > split_folio() > folio_expected_ref_count(folio) != folio_ref_count(folio) - 1 > -EAGAIN > > CPU1 cannot drop its reference until it gets the lock CPU0 holds, so CPU0's > split always fails. folio_trylock() makes CPU1 leave without ever taking a > reference. The PTE branch of this same function already does this, as do > madvise_free_pte_range() and madvise_free_huge_pmd(). > > Reproducer: 400 rounds of eight threads calling MADV_COLD on half of each > of eight THPs, re-formed with MADV_COLLAPSE between rounds. From > /proc/vmstat: > > thp_split_page thp_split_page_failed > before 3186 860 > after 3200 0 > > The short before count is rounds where every thread failed and the > advice was dropped for that THP entirely. > > On failure the walker returns 0 and nothing retries. The PMD path becomes > best effort when the folio lock is held elsewhere - same as the PTE path. Can this result in more EAGAINs being returned to userspace? > --- a/mm/madvise.c > +++ b/mm/madvise.c > @@ -419,9 +419,10 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, > if (next - addr != HPAGE_PMD_SIZE) { > int err; > > + if (!folio_trylock(folio)) > + goto huge_unlock; > folio_get(folio); > spin_unlock(ptl); > - folio_lock(folio); > err = split_folio(folio); > folio_unlock(folio); > folio_put(folio); Sashiko shares my reaction, "dear god, why does that function exist". https://sashiko.dev/#/patchset/20260912110540.3203010-1-gourry@gourry.net Like the entire function, that random mix of "goto foo" with "return whatever" needs to die. Anyway, please check it out while you're on a roll.