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b=U37bawTX8UJU2iy9qXdm6B9yw7N6+y8LKL2hpL/2a4wRUqRCCff7JpdXiBT6ZGDi3 fKcELR9dEua8h5+ndQSAVcAK0AjTAfqNxmsMs/+bag8AvPg1gFoX5NsPsR0iVFJ2Se IgJcdtji09oWqQ5EVQQKpRYLkBQoj4yLK+udxeQLf2hZW5Bu/Gx6uNdjpBhc0OuvwQ XQ5asT7Bv9eqLrzo+zee2wfKW8Jj7koruuSe9X+oPbXCM8Ghabee49lhDw2rey05+Y FAzX9Re1KvMMbZygqHSNLEgVg2VFPI1Ln1gaawgTf0o75xgLtazaplF+YIO7odWJiG q4IdO76V8w38A== Message-ID: Date: Wed, 1 Apr 2026 11:44:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/migrate: fix stale partially_mapped arg to deferred_split_folio() To: Deepanshu Kartikey , akpm@linux-foundation.org, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com Cc: usama.arif@linux.dev, richard.weiyang@gmail.com, sj@kernel.org, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com References: <20260401084116.22219-1-kartikey406@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: ohehkhbmk4uk5tnohg436taxc493dx3e X-Rspamd-Queue-Id: B1A1CC0008 X-Rspam-User: X-Rspamd-Server: rspam03 X-HE-Tag: 1775036673-684502 X-HE-Meta: U2FsdGVkX19xJ2cRQI0gW/b27fm4UySGIRoMI7APVvjhy91TnfJTo5p2hxmqokWVAoFj+y4t42D7Lli7duMcOyhoYxAPkRIpKZfNwlJ/07FPqYNzAj1RgpzLZY9FQ+TNYZku4+GqzXN8E2XadJ07aNBQAqVtJ1dmucckHcQwSPfwA3dC3kVatdcQjIYQT33qjCYr8kPzNh0XEGgHXuTtigMVT27teaAKl8H0dgvF8sFOUG5kWFowBsCzIZcTqncbob7RVGK6PeHbigqWOt3rP1KTIVS61XeBPM1e6mtCvVqtnEOCXVWIhJHOsym/KooEMr61S79Gw/J/h1u/3FZJDqOYWn3LC3kuN6D6k0lbs9Ipm7nKoXHgACtHr/OKMyl1n5eGjbXLxQva1GY3nF1KLXjO8Y351iiGg33f/DlUx1hJ4wOWDHoNtArGnAGuPm/YAyaUfgNhSVrce1beKKJBhQlRICE+7qGj7VtbNZ0GYwr0l0B2gEoH8R5NhjFv8w1PdiC7pDNvKek457P0pKBt0x33CgTkMpT5plTRpQ7fn0NY4UdLAEmh02rDglbgiVdzqQw2VXmNU5hlsx1HZC+1sJEnzWpbxMDcT5EDAz5VDfg0A5dHeVZW64pclQ3COfx8PrcDBwIChEqHv4E6eNi3hmz7Iu2EU2WBqbxjNpOyorWACtpg3t+R55QMlnXlYYq8Z0cciNjmNNZRrzJ+lssjwsCobDrt4c32XUDyKxecyL7eTvumM7VC2yjPpR7TpKyet45TnhV/Ix4bzgDwZ4GDXtYpJ/ejUCJWA9dJHlkk7nn5ts2KngsYdQvYzhtmTZvwq0YXRt32p3ofOAM6iCcNvLIxbFcoWAkhtO8EMxJ9CZ4eK2M4s+VPQX36gfzQkpW8F6qJgVD57RX1PP2SrVrU88O6dGDE869mG7Hg6rmXCZaVNCz1HfQNuBq4uiWhlq0ZSR+30vgbzac0n70Q2ch SyaFCr2H 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/1/26 10:41, Deepanshu Kartikey wrote: > In migrate_folio_move(), src_partially_mapped is sampled from the source > folio before move_to_new_folio() is called: > > if (folio_order(src) > 1 && > !data_race(list_empty(&src->_deferred_list))) { > src_deferred_split = true; > src_partially_mapped = folio_test_partially_mapped(src); > } > > A concurrent thread can unmap pages from the source folio between this > read and the actual migration, making the sampled value stale. Indeed. We could see "partially_mapped" getting set concurrently. > > After move_to_new_folio() succeeds, __folio_migrate_mapping() has already > copied all folio flags including PG_partially_mapped from src to dst. > Passing the stale src_partially_mapped=false to deferred_split_folio(dst) > while dst already has PG_partially_mapped=true triggers the invariant > check in deferred_split_folio(): > > VM_WARN_ON_FOLIO(folio_test_partially_mapped(folio), folio) > > at mm/huge_memory.c:4371, because the argument contradicts the flag > already set on the folio. > > Fix this by removing the src_partially_mapped variable entirely and > reading PG_partially_mapped directly from dst after move_to_new_folio() > completes, where it is authoritative and race-free. > > Fixes: 8a8ca142a488 ("mm: migrate: requeue destination folio on deferred split queue") > Reported-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=a7067a757858ac8eb085 > Tested-by: syzbot+a7067a757858ac8eb085@syzkaller.appspotmail.com > Signed-off-by: Deepanshu Kartikey > --- > mm/migrate.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/mm/migrate.c b/mm/migrate.c > index 05cb408846f2..11236779e910 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1361,7 +1361,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, > int old_page_state = 0; > struct anon_vma *anon_vma = NULL; > bool src_deferred_split = false; > - bool src_partially_mapped = false; > struct list_head *prev; > > __migrate_folio_extract(dst, &old_page_state, &anon_vma); > @@ -1378,7 +1377,6 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, > if (folio_order(src) > 1 && > !data_race(list_empty(&src->_deferred_list))) { > src_deferred_split = true; > - src_partially_mapped = folio_test_partially_mapped(src); > } > > rc = move_to_new_folio(dst, src, mode); > @@ -1404,11 +1402,13 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, > /* > * Requeue the destination folio on the deferred split queue if > * the source was on the queue. The source is unqueued in > - * __folio_migrate_mapping(), so we recorded the state from > - * before move_to_new_folio(). > + * __folio_migrate_mapping(). Read PG_partially_mapped directly from > + * dst: move_to_new_folio() copies all flags from src to dst, so dst > + * now holds the correct authoritative state. Where are we copying PG_partially_mapped exactly? I don't see that happening in folio_migrate_flags(). Also, I wonder what happens if the folio gets partially mapped after already installing migration entries (zapping migration entries). This is all extremely shaky. -- Cheers, David