From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) (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 DD590192D97 for ; Wed, 4 Mar 2026 23:28:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772666934; cv=none; b=raMpPjL1Akdh2+dxCeMV2wht6KSVvkSdvxBDGR5hmfB1DoY6lXX7loNTdAuaHTRNPFF/9y7Jb9jrf6anlgNF2fpDrP1Iz0EH2XcOscRYxnKGemTnblxCDwclhr/t8buEGLyDeoTVugKogbU9YsxNgbIjDtHAGKeGwy7HmtFyHmU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772666934; c=relaxed/simple; bh=zovTW+2shwSOxMFX+IgTvl7P3/St9igE8I4FAthm9mk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Bj34kNdWcLjjwCmfZ6szlAUDd3W32wbxwOZ6uGrECQ+qp6fRy+NefnGCIc3kltbYje4oCQvAzpIIGdzw1E0mDRM3yuEHvKv/0hI8euRLyOv9jqyqUzK1TpnjwJt/m2aG7wqmx1vCvexeRbMn1dlan3xJKcl8w2CFBRCAPsszA6s= 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=OBI9KBjL; arc=none smtp.client-ip=95.215.58.177 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="OBI9KBjL" Message-ID: <622eb392-8c04-473d-b42a-ecdc489799c4@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1772666929; 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=DAFPQJJJhNNoX80Afk7miOJW6kwTQgtVOuUbkrrWT+s=; b=OBI9KBjLrmi5VYZfc3gzIGcvtNnNZJlbIuq6ALvB8cxIR4AuzraUE8t+prekSZFZGMLARc P9oelZQAMs/gueKwMI/QQRS+n2VIZSlpnrb6/17T9L8+he0kY3pFG+AiGqTZ/+uJIzbKl9 NiGBOgYBeSgFmkd/BpdGhDX4+pdlVW8= Date: Thu, 5 Mar 2026 02:28:39 +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: 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, kas@kernel.org, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260304120132.3973445-1-usamaarif642@gmail.com> <5e59c077-9f06-4e45-86e1-ca696e6105b4@nvidia.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: <5e59c077-9f06-4e45-86e1-ca696e6105b4@nvidia.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT 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