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b=MsvhQmXQIGWOrEzkOB51RjEoo39BuIDGE1NPay9656HboXuXmQSLdbZozLFiLCvrf BZZqW6CszlQoISdvqtY4WaLpZ7SF8KvWUZXY054ISW9vLJP1lxsfzeZ92klvXTXLze RycIVmnxTd99DauFgP286aJ9s3tBNtzFgjeVagNb4jlKcdVFKN5sLZ7IUIhLIhC8mf v/wztbvBe/xIF7PrepIP5bFiQUhCoUKkvzuHq5Ac+qMW/WOAfzgVtRe+bRfW7tHAHK NknpWk/iYaugx9Gt8lLOCS3BAlqur/iN0En9YTd8+OOVXce18QqEOHm908EU7VmFkO bnPpWv/86smbQ== Message-ID: Date: Mon, 22 Jun 2026 11:16:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm: migrate: requeue destination folio on deferred split queue To: Wei Yang , Usama Arif Cc: 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 References: <20260310105419.3256755-1-usama.arif@linux.dev> <20260620072721.x4dfh6gy4wnjqre4@master> 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: 4jy3rhhss586m7mqy8y6gfwcfesukew3 X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 0D1A720002 X-Rspam-User: X-HE-Tag: 1782119805-942699 X-HE-Meta: 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 wU3kTnia 1KuJUG9ev9AGPsjH2X3wKpUPiGI2C0+q921njcze6LLbTKXtrWaSw6oWHMzSmDDmaWJdBv23K9mW0j89NlPT5aQAs1Fnw2lj0kbnGGJh3Fwrsx5wWox0md7lHb1+5uShq1+BoOFQ5bvVUGQRgewLZT78KRtklqtTMfj/qtssIy3mp7VMwbt1cpPUGbNfSyvc6awuHsCS4/GVhaj29O63HE5AEg72kJ8hHe4prEKUNaXY5OKUzlUjqVP5hgUYvBNeQfm+a7f1MlzPeifseUcHwwVxFGCXPcVaaJd9n+Vvge28PppkBlEBa0z/o8iMvTXwCBsBXi+gd3BgZSySmKpRBbpQTVac11YO0psCzmrYDRb9JgsP5Wg3Y8wHhThQnrzhj2dtl8x1H1f9g6JE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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? -- Cheers, David