From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) (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 6C1911386C9 for ; Mon, 9 Mar 2026 21:35:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773092104; cv=none; b=GU9cCllqrYIxWtDAUmI2/Q1cAIHLpArN0u2BTLBx0klXiftIIY2/lceTw7QFK7gOBWfY7MMDZ/QXaR8d4uOe7T3B18W/xTHjKunrsvYVFbMc08RT0XXV8S3Wp2HfNLtTIq2z3rTua77L1ujrBzLyIicdeTPaUCW4SamH4Wh/CIk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773092104; c=relaxed/simple; bh=uYcQXQANvQYncDdLBKUkt285zxtjr+wSPpd5cv6Jknc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ROFRyWxpthCfFuzyDk9u7agOtyh8bX+6jtZTgizo5NtqT+gPPq2zTQlocoSqK6NyXQ6yllRqadLj9kP5bluLIUNor/H4PFUCd5Zn8XTqsVjhCkWXVcNw6wb3kEMgQqZj5IsWm7B4t6mBAa2vc5OSCWgGVAVthwkdTrJCIlzvuuQ= 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=UfIiuqUj; arc=none smtp.client-ip=91.218.175.189 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="UfIiuqUj" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1773092101; 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=9Z89Z0CbGlfYyK9enL1aU6uEk1uYUJc2hi9of77FKCc=; b=UfIiuqUj5RN4OnUp4DNTD94bNDfHXU/WZAsxKKJI+0HRS89NtQj96WZrCoQLc2+MRaJK9s E3IB3pqneljaxU6hi6H97Os7emjBXks+52uiFFqGOY8CELSXzuwd1DUHfPtD1cG6LsElAj bpL91EN1nFg/AuG8ETeRKDtWEndcmAs= Date: Tue, 10 Mar 2026 00:34:39 +0300 Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [RFC v2 12/21] mm: thp: handle split failure in device migration Content-Language: en-GB To: Nico Pache Cc: Andrew Morton , david@kernel.org, lorenzo.stoakes@oracle.com, willy@infradead.org, linux-mm@kvack.org, fvdl@google.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, maddy@linux.ibm.com, mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, linux-s390@vger.kernel.org References: <20260226113233.3987674-1-usama.arif@linux.dev> <20260226113233.3987674-13-usama.arif@linux.dev> <6982e9fc-cc17-4d4f-b26e-83997c4bf070@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 09/03/2026 18:09, Nico Pache wrote: > On Thu, Mar 5, 2026 at 9:55 AM Usama Arif wrote: >> >> >> >> On 02/03/2026 21:20, Nico Pache wrote: >>> On Thu, Feb 26, 2026 at 4:34 AM Usama Arif wrote: >>>> >>>> Device memory migration has two call sites that split huge PMDs: >>>> >>>> migrate_vma_split_unmapped_folio(): >>>> Called from migrate_vma_pages() when migrating a PMD-mapped THP to a >>>> destination that doesn't support compound pages. It splits the PMD >>>> then splits the folio via folio_split_unmapped(). >>>> >>>> If the PMD split fails, folio_split_unmapped() would operate on an >>>> unsplit folio with inconsistent page table state. Propagate -ENOMEM >>>> to skip this page's migration. This is safe as folio_split_unmapped >>>> failure would be propagated in a similar way. >>>> >>>> migrate_vma_insert_page(): >>>> Called from migrate_vma_pages() when inserting a page into a VMA >>>> during migration back from device memory. If a huge zero PMD exists >>>> at the target address, it must be split before PTE insertion. >>>> >>>> If the split fails, the subsequent pte_alloc() and set_pte_at() would >>>> operate on a PMD slot still occupied by the huge zero entry. Use >>>> goto abort, consistent with other allocation failures in this function. >>>> >>>> Signed-off-by: Usama Arif >>>> --- >>>> mm/migrate_device.c | 16 ++++++++++++++-- >>>> 1 file changed, 14 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c >>>> index 78c7acf024615..bc53e06fd9735 100644 >>>> --- a/mm/migrate_device.c >>>> +++ b/mm/migrate_device.c >>>> @@ -909,7 +909,13 @@ static int migrate_vma_split_unmapped_folio(struct migrate_vma *migrate, >>>> int ret = 0; >>>> >>>> folio_get(folio); >>> >>> Should we be concerned about this folio_get? Are we incrementing a >>> reference that was already held if we back out of the split? >>> >>> -- Nico >> >> >> >> Hi Nico, >> >> Thanks for pointing this out. It spun out to an entire investigation for me [1]. > > Hey Usama, > > I'm sorry my question lead you down a rabbit hole but I'm glad you did > the proper investigation and found the correct answer :) Thanks for > looking into it and for clearing that up via a comment! > > Cheers, > -- Nico > Thanks for the review! There is a need for folio_put in this patch specifically for split_huge_pmd_address which I wouldnt have figured out without your comment, so really appreciate it! Usama