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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 May 2025 08:58:16.1369 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 665e7e6f-caa9-4595-af33-08dd9cfc9e8f X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB5PEPF00014B98.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PA4PR08MB5885 X-Stat-Signature: dj67nxhwez89336p49t9r7o61z8y3zic X-Rspamd-Queue-Id: AE031140014 X-Rspam-User: X-Rspamd-Server: rspam02 X-HE-Tag: 1748336300-359826 X-HE-Meta: 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 lWavSevU 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 27/05/25 2:10 pm, David Hildenbrand wrote: > On 27.05.25 10:06, Dev Jain wrote: >> >> On 27/05/25 1:18 pm, David Hildenbrand wrote: >>> On 27.05.25 05:20, Dev Jain wrote: >>>> >>>> On 26/05/25 11:58 pm, Shivank Garg wrote: >>>>> hpage_collapse_scan_file() calls is_refcount_suitable(), which in >>>>> turn >>>>> calls folio_mapcount(). folio_mapcount() checks folio_test_large() >>>>> before >>>>> proceeding to folio_large_mapcount(), but there is a race window >>>>> where the >>>>> folio may get split/freed between these checks, triggering: >>>>> >>>>>      VM_WARN_ON_FOLIO(!folio_test_large(folio), folio) >>>>> >>>>> Take a temporary reference to the folio in >>>>> hpage_collapse_scan_file(). >>>>> This stabilizes the folio during refcount check and prevents >>>>> incorrect >>>>> large folio detection due to concurrent split/free. Use helper >>>>> folio_expected_ref_count() + 1 to compare with folio_ref_count() >>>>> instead of using is_refcount_suitable(). >>>>> >>>>> Fixes: 05c5323b2a34 ("mm: track mapcount of large folios in single >>>>> value") >>>>> Reported-by: syzbot+2b99589e33edbe9475ca@syzkaller.appspotmail.com >>>>> Closes: >>>>> https://lore.kernel.org/all/6828470d.a70a0220.38f255.000c.GAE@google.com >>>>> >>>>> >>>>> Suggested-by: David Hildenbrand >>>>> Acked-by: David Hildenbrand >>>>> Signed-off-by: Shivank Garg >>>>> --- >>>> >>>> The patch looks fine. >>>> >>>> I was just wondering about the implications of this on migration. >>>> Earlier >>>> we had a refcount race between migration and shmem page fault via >>>> filemap_get_entry() >>>> taking a reference and not releasing it till we take the folio lock, >>>> which was held >>>> by the migration path. I would like to *think* that real workloads, >>>> when migrating >>>> pages, will *not* be faulting on those pages simultaneously, just >>>> guessing. But now >>>> we have a kernel thread (khugepaged) racing against migration. I may >>>> just be over-speculating. >>> >>> I'm not quite sure I understand the concern you have. Any temporary >>> reference can temporarily block migration, however, the retry logic >>> should be able to handle that just fine -- and this code is not really >>> special (see filemap_get_entry()). >> >> >> You are correct that any temp ref can block migration, however, that >> reference has to come after the folios have been isolated in the >> migration path. >> >> So the probability of someone taking a reference on the folio is quite >> low since it has been isolated. The problem with filemap_get_entry() is >> that it finds >> >> the folio in the pagecache, so isolation is useless, then takes a >> reference and then shmem_get_folio_gfp() does a folio_lock() instead of >> folio_try_lock(). >> >> This was the race which I talked about an year back at [1]. My concern >> is that we are adding another candidate to that race; just wondering if >> there is >> >> a better solution to fix the race mentioned in Shivank's patchset. > > Note that in this code here, we're not locking the folio just yet, > we're only grabbing a reference for a very short time. Ah now I notice that the reference is taken for a very short time. > > We will isolate+lock the folios in collapse_file(), where we also lock > all slots in the pagecache. > > The alternative would be to also lock all slots here, which is > arguably worse. Makes sense. Acked-by: Dev Jain