From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-182.mta1.migadu.com (out-182.mta1.migadu.com [95.215.58.182]) (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 4928F2749ED for ; Thu, 5 Mar 2026 11:44:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772711070; cv=none; b=iF3segcQDJi/WUyxWrT/89ZnkSbGTxRMteiT+0p7OeXDwdkhpOP2juUV5U91iMQ1/JxACs3RclXBNVD6ilUuKN4UJobhgYbzwzIZax10LUE5b7bzpoK+wzUjqb4ClXW63ed5tXpYroOB+60egJRjfJooMY8YEmcBzBvYXSkfq3k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772711070; c=relaxed/simple; bh=DD3mL8f27jDDqujVGbvEhjMkb8QR6wjaEBqkrhPlgCU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rfSOnQKZT0DZlSskN3AHBSvGjQT+oNVj18OiAYdZOEoLQUM6g8z6JtCXJ/iuwQ9LUHg5fh7AL8QzXS0RqCQ2h/n7qtMWEdI98iojD+sUWXShotZMh3nwhzlZQ7V+SBX5mSR4HTyAM9kIpQrrLB3qrGUC5Kksm41HYUvYSYQaMQQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=a/qWqMU5; arc=none smtp.client-ip=95.215.58.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="a/qWqMU5" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772711066; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ddRQSUpzR8bRhlhTQBip0BDAIGV0lwUiZeArCVwecRs=; b=a/qWqMU5Yf8yyNjXfk6Oy/9D+nDcKqmO370hdBQqe9JXom5N+/9LDQn4n9ZOErIv6t2Qkw CUZQ1QFcyU0jnb1ZR1rQgtxNyp2RyFhppjTOUAG5+clkomGUjL8W8xtHg9lxfYX6kSugiI VNrMwzqeYp5Vz0FTDrzgLcWZrQevdPs= Date: Thu, 5 Mar 2026 14:44:20 +0300 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH] mm/migrate_device: fix folio refcount leak on folio_split_unmapped failure Content-Language: en-GB To: =?UTF-8?Q?Mika_Penttil=C3=A4?= , Balbir Singh , Zi Yan , Kiryl Shutsemau , matthew.brost@intel.com, npache@redhat.com, david@kernel.org Cc: Usama Arif , Andrew Morton , linux-mm@kvack.org, 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, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260304120132.3973445-1-usamaarif642@gmail.com> <5e59c077-9f06-4e45-86e1-ca696e6105b4@nvidia.com> <622eb392-8c04-473d-b42a-ecdc489799c4@linux.dev> <942f2df4-6fb5-415f-b7d4-87a83315890b@redhat.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: <942f2df4-6fb5-415f-b7d4-87a83315890b@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 05/03/2026 06:09, Mika Penttilä wrote: > Hi! > > On 3/5/26 01:28, Usama Arif wrote: > >> >> On 04/03/2026 22:09, Balbir Singh wrote: >>> On 3/5/26 08:54, Zi Yan wrote: >>>> On 4 Mar 2026, at 16:48, Balbir Singh wrote: >>>> >>>>> On 3/5/26 02:17, Zi Yan wrote: >>>>>> On 4 Mar 2026, at 7:01, Usama Arif wrote: >>>>>> >>>>>>> From: Usama Arif >>>>>>> >>>>>>> migrate_vma_split_unmapped_folio() takes an extra reference via >>>>>>> folio_get() before calling folio_split_unmapped(). On success, the >>>>>>> split consumes this reference: __folio_freeze_and_split_unmapped() >>>>>>> expects the +1 in its folio_ref_freeze() check, and distributes it >>>>>>> across the resulting sub-folios via folio_ref_unfreeze(...+1), which >>>>>>> are later balanced by folio_put() calls in __migrate_device_finalize(). >>>>>>> >>>>>>> If folio_split_unmapped() fails (e.g., unexpected pinning returns >>>>>>> -EAGAIN), the function returns without calling folio_put(). The extra >>>>>>> reference is never released. >>>>>>> >>>>>>> Add the missing folio_put() on the error path. >>>>>>> >>>>>>> Fixes: 4265d67e405a4 ("mm/migrate_device: add THP splitting during migration") >>>>>>> Closes: https://lore.kernel.org/all/CAA1CXcDyqPPwf_-W7B+PFQtL8HdoJGCEqVsVxq7DhOUB=L4PQA@mail.gmail.com/ >>>>>>> Reported-by: Nico Pache >>>>>>> Signed-off-by: Usama Arif >>>>>>> --- >>>>>>> mm/migrate_device.c | 4 +++- >>>>>>> 1 file changed, 3 insertions(+), 1 deletion(-) >>>>>>> >>>>>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>>>>> index 0a8b31939640f..351ecd9065d13 100644 >>>>>>> --- a/mm/migrate_device.c >>>>>>> +++ b/mm/migrate_device.c >>>>>>> @@ -917,8 +917,10 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, >>>>>>> folio_get(folio); >>>>>>> split_huge_pmd_address(migrate->vma, addr, true); >>>>>>> ret = folio_split_unmapped(folio, 0); >>>>>>> - if (ret) >>>>>>> + if (ret) { >>>>>>> + folio_put(folio); >>>>>>> return ret; >>>>>>> + } >>>>>>> migrate->src[idx] &= ~MIGRATE_PFN_COMPOUND; >>>>>>> flags = migrate->src[idx] & ((1UL << MIGRATE_PFN_SHIFT) - 1); >>>>>>> pfn = migrate->src[idx] >> MIGRATE_PFN_SHIFT; >>>>>>> -- >>>>>>> 2.47.3 >>>>>> Add Balbir, who wrote the code, to comment on this. >>>>>> >>>>> Thanks Zi! >>>>> >>>>> Just wondering if there is a reproducer for the issue and how the fix was tested? >>>>> I expect migrate_vma_finalize() to be called for folios, even when split failed and >>>>> drop the lock. >>>> Does migrate_vma_finalize() do folio_put() for failed-to-split folios? >>>> If so, how does it distinguish between split folios and failed-to-split folios? >>>> By comparing source and destination folio orders? >>>> >>> We reset the MIGRATE_PFN_MIGRATE flag for failing to migrate pfns. We do a folio_put >>> on the src in finalize, if it is split then on all the split folios as well. >>> >>>> What we see from migrate_vma_split_unmapped_folio() is that >>>> it adds a refcount for all input folios, but only drops a refcount >>>> for the split folio. Isn’t it cause failed-to-split folios to have >>>> additional refcount? >>>> >> Hello! >> >> Thanks for reviewing everyone. So its very difficult to create a reproducer I think >> the extra reference would need to appear after migrate_device_unmap() but before >> folio_split_unmapped() in migrate_vma_pages()? That's hard to trigger reliably from >> userspace. >> >> The fix came about when Nico indicated there might be an issue if split_huge_pmd_address >> fails in my patch [1]. >> >> Below is my understanding of how refcounting is working over here step by step. I >> might very well be wrong on this, and the refcounting is a bit all over the place >> and I might miss a reference change somewhere so would really appreciate if someone >> can confirm this! >> >> >> 1. migrate_vma_collect_huge_pmd(): >> a) folio_get(folio) -> +1 (collect reference) >> 2. migrate_device_unmap(): >> a) folio_isolate_lru() -> +1 (isolation reference) >> b) folio_put() -> -1 (drops the collect reference) >> >> >> Without this patch fix: >> >> 3. migrate_vma_split_unmapped_folio(): >> a) folio_get(folio) -> +1 (split reference) >> b) folio_split_unmapped() -> fails >> c) Returns error — without folio_put() which is the fix >> 4. Caller in migrate_vma_pages(): clears MIGRATE_PFN_MIGRATE | MIGRATE_PFN_COMPOUND >> 5. __migrate_device_finalize(): sees !(src_pfns[i] & MIGRATE_PFN_MIGRATE), restores the folio: >> a) remove_migration_ptes(src, src) — re-establishes user PTEs >> b) folio_unlock(src) >> c) folio_put(src) -> -1 (drops the isolation reference) >> >> The split reference in 3.a is never released and the folio has a permanently elevated refcount. >> Unless I missed a folio_put somewhere for the refcount increase in folio_isolate_lru() (2.b)? >> >> Please let me know if this makes sense! >> >> [1] https://lore.kernel.org/all/CAA1CXcDyqPPwf_-W7B+PFQtL8HdoJGCEqVsVxq7DhOUB=L4PQA@mail.gmail.com/ >> >>> Thanks! Yes, the patch makes sense >>> >>> Acked-by: Balbir Singh >>> >>> Balbir > > I remember stumbling on this while ago also. The folio_get() in migrate_vma_split_unmapped_folio() > is balanced with put_page() in __split_huge_pmd_locked() (freeze = true), can't fail for device pages. > Folios at this point are unmapped but have 1 refcount from "collecting". > After folio_split_unmapped() the refcount(s) is still 1. > > So it seems the code is good as is? A comment though would be good for the extra folio_get.. > hmm I dont think the put_page() in __split_huge_pmd_locked() is there to balance the folio_get() in migrate_vma_split_unmapped_folio(). There are other points where split_huge_pmd_locked() is called with freeze = true [1] and they don't get a reference before calling split_huge_pmd. I think the folio_put() in __split_huge_pmd_locked() freeze = true case is there as migration entries are being installed? [1] https://elixir.bootlin.com/linux/v6.19.3/source/mm/rmap.c#L2334