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Mon, 22 Jun 2026 06:43:32 -0700 (PDT) Date: Mon, 22 Jun 2026 13:43:31 +0000 From: Wei Yang To: "David Hildenbrand (Arm)" Cc: Wei Yang , Usama Arif , Andrew Morton , npache@redhat.com, ziy@nvidia.com, willy@infradead.org, linux-mm@kvack.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, hannes@cmpxchg.org, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v2] mm: migrate: requeue destination folio on deferred split queue Message-ID: <20260622134331.xvbpu6edwml65myp@master> Reply-To: Wei Yang References: <20260310105419.3256755-1-usama.arif@linux.dev> <20260620072721.x4dfh6gy4wnjqre4@master> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) X-Rspamd-Queue-Id: 96C24140002 X-Rspam-User: X-Rspamd-Server: rspam09 X-Stat-Signature: 9dwkr1brprthqfdwhia3bnftmprggwix X-HE-Tag: 1782135814-416054 X-HE-Meta: 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 wyWSeHPm 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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 Mon, Jun 22, 2026 at 11:16:39AM +0200, David Hildenbrand (Arm) wrote: >On 6/20/26 09:27, Wei Yang wrote: >> On Tue, Mar 10, 2026 at 03:54:19AM -0700, Usama Arif wrote: >>> During folio migration, __folio_migrate_mapping() removes the source >>> folio from the deferred split queue, but the destination folio is never >>> re-queued. This causes underutilized THPs to escape the shrinker after >>> NUMA migration, since they silently drop off the deferred split list. >>> >>> Fix this by recording whether the source folio was on the deferred split >>> queue and its partially mapped state before move_to_new_folio() unqueues >>> it, and re-queuing the destination folio after a successful migration if >>> it was. >>> >>> By the time migrate_folio_move() runs, partially mapped folios without a >>> pin have already been split by migrate_pages_batch(). So only two cases >>> remain on the deferred list at this point: >>> 1. Partially mapped folios with a pin (split failed). >>> 2. Fully mapped but potentially underused folios. >>> The recorded partially_mapped state is forwarded to deferred_split_folio() >>> so that the destination folio is correctly re-queued in both cases. >>> >>> Reported-by: Johannes Weiner >>> Fixes: dafff3f4c850 ("mm: split underused THPs") >>> Signed-off-by: Usama Arif >>> --- >>> v1 -> v2: >>> - record whether source folio was on the deferred split queue before >>> move_to_folio() (David) >>> - record partially mapped state and update commit message (Zi) >>> --- >>> mm/migrate.c | 17 +++++++++++++++++ >>> 1 file changed, 17 insertions(+) >>> >>> diff --git a/mm/migrate.c b/mm/migrate.c >>> index ece77ccb2ec0..61013d258eb4 100644 >>> --- a/mm/migrate.c >>> +++ b/mm/migrate.c >>> @@ -1360,6 +1360,8 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, >>> int rc; >>> 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); >>> @@ -1373,6 +1375,12 @@ static int migrate_folio_move(free_folio_t put_new_folio, unsigned long private, >>> goto out_unlock_both; >>> } >>> >>> + if (folio_test_large(src) && folio_test_large_rmappable(src) && >>> + !data_race(list_empty(&src->_deferred_list))) { >>> + src_deferred_split = true; >>> + src_partially_mapped = folio_test_partially_mapped(src); >>> + } >> >> Hi, Usama >> >> I am afraid there maybe a race between migration and defer_split. >> >> A B >> migrate_pages_batch deferred_split_scan >> migrate_folio_unmap list_del_init(&folio->_deferred_list) >> folio_lock/folio_trylock >> >> migrate_folios_move >> migrate_folio_move >> list_empty(&src->_deferred_list) >> folio_trylock() >> requeue: >> >> In case list_empty() check happens after folio removed from defer_list but >> before requeued, we will miss this folio. > >deferred_split_isolate() would grab a reference through folio_try_get(). > >How can we migrate a folio with a raised refcount? > Thanks, I missed expected_refcount check in __migrate_folio(). >-- >Cheers, > >David -- Wei Yang Help you, Help me