From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 9A64F139D for ; Mon, 9 Mar 2026 20:22:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773087777; cv=none; b=Chiv2zFIFoqW6ZwpjnhzKZw4cO3M/siQ1GINXA7VjAOPBJJlJmNse0skxbd0uAiUYteIamNT3vqnerzkaqMwfMBH0aCv/niDtRDV/FMzrWGf2YjsqaIcs/OvZyv8N+2GWYq+8nxvrZ6EyGvvmNZuH3EzvcQZHEgO4R1OlJrgOzE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773087777; c=relaxed/simple; bh=fSaRukjxWJ92UeSz7ouhJj6LJujaYf3lOtbd8VWs+TA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=DEwXBQvmyU9ZqqnRzuusJRLvdsd3rLSyTCwg89uNk0ZWctleDrwvIvaLLkKep2st1xZnxHQ+jfmsEewUgIo3/SurGvLnlsZKOREvzS4NrwRvQBi4pyX6RQDStu9ThKkq0Kzn8tOliqPQdOQT9u0KweiSpovO4+dVxQsrP2LNhrA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=B7IXhROP; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="B7IXhROP" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1D480C4CEF7; Mon, 9 Mar 2026 20:22:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773087777; bh=fSaRukjxWJ92UeSz7ouhJj6LJujaYf3lOtbd8VWs+TA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=B7IXhROP5eZaXsujy91kFeFuniY2+1W+nwoEZozilAP2brfi+8rz1JbIlV6ol7P+p V6t5pR+3rlrrkGCvtbFw9bEEbbZQBxsm7lmTJr7vcpBareDfr6A4WlSwQNc/UwIEiO qnBl+RjBDaOPCF0hnKp0+P9NWjWc9zS9IvOE4KSGmx71BRJpo+kBm/PvY2sOalQtya 6XVS2O9/7+xXHk8awhoAnFPDfM5SRQUBzZ8F+p/WSAC8QFFZ477HgjcCjxQ/Vu6+sE M5np1cHKF+kjVfZf97LZP0j4P84yuGzmpX+fX4034G+TyFdcVAR7OmnUD1OroDGCbF XCZleASryZAJg== Message-ID: <57cf14b7-e5cc-4298-9d56-978b855e3276@kernel.org> Date: Mon, 9 Mar 2026 21:22:47 +0100 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] mm/migrate_device: document folio_get requirement before frozen PMD split To: Usama Arif , Andrew Morton , npache@redhat.com, ziy@nvidia.com, linux-mm@kvack.org, mpenttil@redhat.com, balbirs@nvidia.com Cc: 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, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260306104409.3915811-1-usama.arif@linux.dev> 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 3/9/26 20:11, Usama Arif wrote: > > > On 09/03/2026 18:18, David Hildenbrand (Arm) wrote: >> On 3/6/26 11:44, Usama Arif wrote: >>> split_huge_pmd_address() with freeze=true splits a PMD migration entry >>> into PTE migration entries, consuming one folio reference in the >>> process. The folio_get() before it provides this reference. >>> >>> Add a comment explaining this relationship and a VM_WARN_ON_ONCE to >>> catch an unexpected refcount != 1 entry state. >>> >>> Suggested-by: Zi Yan >>> Signed-off-by: Usama Arif >>> --- >>> mm/migrate_device.c | 5 +++++ >>> 1 file changed, 5 insertions(+) >>> >>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>> index 78c7acf024615..6fa2878848a7e 100644 >>> --- a/mm/migrate_device.c >>> +++ b/mm/migrate_device.c >>> @@ -908,6 +908,11 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, >>> unsigned long flags; >>> int ret = 0; >>> >>> + VM_WARN_ON_ONCE(folio_ref_count(folio) != 1); >> >> Can't we have speculative references here? In general, asserting that >> the refcount has an exact value (besides 0) is often shaky. > > > I hope not at this point in code. > > At this point, the folio is locked and unmapped (both done in migrate_vma_collect_huge_pmd()), > and the present PMD was set to migration entry. It is isolated from LRU in > migrate_device_unmap(). So the folio should not be visible to GUP or reclaim/compaction. > Only anon, non-swapcache folios should reach here. So it won't run into any folio_try_get > in page cache or swap cache. We have other pfn walkers that can just temporary grab a reference, to immediately back off. So it's not very reliable to depend on that. > > The folio_get() done in migrate_vma_split_unmapped_folio() is consumed by > split_huge_pmd_address(), and folio_split_unmapped() expects a folio_reference > of 1 after this [1]. > > If its not considered good to assert a non zero refcount value, I can change the > warning to a comment, but I think refcount should be 1 at this point, otherwise > folio_split_unmapped will fail. Well, yes. folio_split_unmapped() will handle this gracefully (as documented), without triggering a warning. So best to remove the VM_WARN_ON_ONCE(). -- Cheers, David