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 E2BC53ABD9D for ; Tue, 28 Jul 2026 12:50:53 +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=1785243055; cv=none; b=u+dXRdN8GYXqIx7bG5tw7jVv5/mg0ow7XquCh5Drst95iAfLBmfxueFAjPy1ybcaiSVadyiPy392/FG/rHYHKDMAHsXJt9MmwGKL4wJ/HboSlINt5YBHM3GASxBWPZtcLXWCAWe6ygS4N7RC3hJ+q2OQuJA4FW+Uq/xO8mZEF5s= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785243055; c=relaxed/simple; bh=KeA5E67cOUR0KKQszSoRfM/iXzxNQKX0m67cZFg/0z0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gTlUgxZYitHhIdYthPE4zCKoD1mT8u5tFtpHfoqWpnAEVT1Wyoh843vaFzdCi5FY8nZnjQsE5JMhltvI+PoGkQ66EKI1AMYt2xevA3PGHMC0AdSUOclbmBaN6ct+P3yeaNGlEhiyz9O0leYIMrYGI0D5dC6ezZw2dO25ChinA8U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=I6esq+r6; 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="I6esq+r6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 19AF51F00A3A; Tue, 28 Jul 2026 12:50:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785243053; bh=Ne1QxTrwUiufj27QLOxN+cEuVrvrZctkzV6bT7vB6lY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=I6esq+r6jOKQTWD243iyapiiUo/D7oSzgNZ/yPfvQVkJrw7NKpAw8Vs2cdvEkHLC6 9dr16Mkre69dNZ5teA7l4qKieFv5QxHvRUlaChOihWHvk+TjTiFCxT73bZDCyV7H7O a4j33KKBPY1HT3D+sbkMiwesFyl9IgoC2Buu2YN9u8ML0bwEge4XZZpl3XbF8HsJJK TJplOrd06l0Txhk1PWSyHI3cTUTlkmePckZOWoNWrfneWt+PyNXDSz0zI++19X5DVQ TkowH7a/aVf4enaTfpg4PiM0geRWXjRJgAltc7sC2tUl7C8tJ+UwM2PE9S7ke0bgwV ycYdjkzKl5tAg== Message-ID: <8d50f94b-6d95-4967-aa1f-2c14333ffd85@kernel.org> Date: Tue, 28 Jul 2026 14:50:46 +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 v4] mm/migrate_device: Clear stale mapping after freeing swapcache To: Arvind Yadav , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, matthew.brost@intel.com, joshua.hahnjy@gmail.com, ziy@nvidia.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, balbirs@nvidia.com References: <20260728062832.1107127-1-arvind.yadav@intel.com> 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 7/28/26 08:28, Arvind Yadav wrote: > __migrate_device_pages() reads the folio mapping before calling > folio_free_swap(). When folio_free_swap() succeeds, the folio is removed > from the swap cache, but the saved mapping still points to swap_space. > > Passing the stale mapping to folio_migrate_mapping() makes it use the > mapped-folio path for a folio that is no longer in swapcache. It can > then operate on swap_space.i_pages with invalid reference accounting, > eventually triggering a folio reference count BUG. > > After a successful split, nr still contains the number of pages in the > original large folio, although each resulting page is now a separate > order-0 folio. Reset nr to 1 so each split folio is processed separately, > including its own swapcache removal and mapping lookup. > > Refresh the saved mapping after folio_free_swap() so the current folio > state is used during migration. > > v2: > - Refresh the mapping using folio_mapping(), as suggested by Zi Yan. > > v3: > - Reset nr to 1 after a successful split so each resulting folio is > processed independently, as suggested by Zi Yan. > > v4: > - Re-read each source folio's mapping immediately before > folio_migrate_mapping(), as suggested by Balbir Singh. > - Add a warning to validate the post-split order-0 invariant, > as suggested by Balbir Singh. > > Fixes: df263d9a7dff ("mm/migrate_device: try to handle swapcache pages") > Cc: Andrew Morton > Cc: David Hildenbrand > Cc: Matthew Brost > Cc: Joshua Hahn > Cc: Rakie Kim > Cc: Byungchul Park > Cc: Gregory Price > Cc: Ying Huang > Cc: Alistair Popple > Reviewed-by: Zi Yan > Reviewed-by: Balbir Singh > Signed-off-by: Arvind Yadav > --- > mm/migrate_device.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 554754eb26ff..0e6f00190fd8 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -1182,6 +1182,13 @@ static void __migrate_device_pages(unsigned long *src_pfns, > MIGRATE_PFN_COMPOUND); > goto next; > } > + > + /* > + * reset nr so that only first after-split folio > + * is processed below > + */ > + VM_WARN_ON_ONCE(folio_test_large(folio)); > + nr = 1; This function is surely a beauty. (had to rephrase that sentence 3 times ;) ) migrate_vma_split_unmapped_folio() modifies the src_pfns() entries on success. (and somehow assumes that it's always a THP, what? After a MIGRATE_PFN_COMPOUND value is set? What? Why the "nr = 1 << folio_order(folio);" in the caller). If we ended up modifying the current entry in such a way, shouldn't we just have retry: label and restart at the very top of the function, where we just naturally re-read the entry/page/folio and do the right thing? > } else if ((src_pfns[i] & MIGRATE_PFN_MIGRATE) && > (dst_pfns[i] & MIGRATE_PFN_COMPOUND) && > !(src_pfns[i] & MIGRATE_PFN_COMPOUND)) { > @@ -1221,6 +1228,12 @@ static void __migrate_device_pages(unsigned long *src_pfns, > folio = page_folio(migrate_pfn_to_page(src_pfns[i+j])); > newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j])); > > + /* > + * folio_free_swap() removed the folio from the swap > + * cache. Refresh the saved mapping before migration. > + */ > + mapping = folio_mapping(folio); > + > r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt); > if (r) > src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE; While this looks good, I do wonder why do we have to supply the mapping here at all? Is there a path where we call folio_migrate_mapping() and the old folio (folio) does no longer have the right mapping attached? It would be a lot less error prone if the function would just obtain the mapping from the old folio. -- Cheers, David