From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E4CC7364926 for ; Mon, 22 Jun 2026 09:16:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782119809; cv=none; b=gNETgdNfi5SRLL3IjLgpBHg0A9zQjuSxC30ciuDYLv2cIdvVhUpJ34yqmxsybn0H0euKsWPn0neBOoZ6LErFa8XnaP6ldu+LL+IIOtz3VJPQyFW2v5pNx8w/Q1zOtG+Gfpjkpn5rikcmoiuGgapc/xhySs4Ezh1n8tEyRooBYME= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782119809; c=relaxed/simple; bh=4PzQZ+1D5MhXwqsxzsfRmKzETppty6oOdER9ZBiCDfo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tZUNuVx3MqRHYNmluTgfes8LcC5TfqBTtl6IMzL1IUQ6pDI/tPRHM9kAaBrorkGGrVZHqxIiZcU1uE/CtmrxEPeKwQixd/PkKJwWHCFDer3td6jsjIWO1c98/UXxu1Xhko5bzsnGQjOML+EySrPKtnAppjn7ouQqu5Gaekr2+tc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=MsvhQmXQ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="MsvhQmXQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id DE4D41F00A3A; Mon, 22 Jun 2026 09:16:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782119805; bh=sTvbDmMhUjwqiFd2kPQ5Mc6EsPrALFmqEPayYyfrbfs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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