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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , David Rientjes , Shakeel Butt , Andrew Morton , Jann Horn , Pedro Falcato , "Matthew Wilcox (Oracle)" , Jan Kara , Nhat Pham , Johannes Weiner , Zi Yan , Matthew Brost , Joshua Hahn , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Usama Arif , Kairui Song , =?utf-8?B?SsOpcsO0bWU=?= Glisse , Kiryl Shutsemau , Yu-cheng Yu , Dave Hansen , Dennis Zhou , Tejun Heo , Christoph Lameter , Peng Fan , Josef Bacik , Chris Mason , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, "Borislav Petkov (AMD)" , Mark Brown , "Dennis Zhou (Facebook)" , Rick Edgecombe , Josef Bacik , stable@vger.kernel.org Subject: Re: [PATCH 04/13] mm/migrate_device: allocate THP deferred split heads Message-ID: References: <20261111-mason-migrate-et-al-fixes-v1-0-3d2454362e8d@kernel.org> <20261111-mason-migrate-et-al-fixes-v1-4-3d2454362e8d@kernel.org> Content-Type: text/plain; charset=us-ascii 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U2FsdGVkX19GnuAMoPy+3kxtpcXpDkRVBwUOYtsGKp19XQIaa74XsUUXQqi3KdvlEQtxWkq3u+UsyndLRaZ7aHW92ha5nuVA3+EAVYqE1JvH/zPy8j+qKMKZuwZVkF6f8Ig2ywwF2Yx40BIexd2gHEZHhLdLhWlQaLicpqUFiX5O93p58hbpAA5LBt9TYJG52w+bqKN6d9cNCxQghp94P4QzN3pHHNHZmEJNkWPGsPlURc+sDwGC/cZTbKor9DBul4rNkDeLolsqqwlnnq174ezyOeTYWWtQy8zTqyXOi+FNY8HkGkObbJ9TR2eoqtxA9E2TLFjd3rZv+waPocJLwajph3+x55IyA7BpMDNYdzuMtgqKu2oAzE7ajXccAOadO6ZfdohQDQxYPK8UbRiLB2ZUhNSCHEX6gx03/dCyoQSkbv257K2IjXhadINFuREaft5VakU4fWlcB+csu2Lbsit0Y40EBUmiPjbxU0DWwnmLJ9pxol/ItYejtwfykNeMIgYC82/4wv/7ZUlxc9qTQvAYO+d3TJK90a1m1Z/BVScXvnt8tlFM6lRU+to4cHhYhmGhn2CyhY/yJGKyjaJJ9OVtZ1vhfIl7uPApOtOWXYvcYD2QVupz9Sfbb4Ub3IP181bpu1NGZb9Z/US322oTdID8t64odVJTY3YYEcLVaWzBtpZG2+EMw0QAoBmy5oqX2tOnD3YMRTJpOkhnDNd5AVYT/IgiVNTEdJMaKw4X73IB/T7rsnNfDH7gONcB+DOraP2n0omY1rL/5ouX165AsBLIWD5vllPT1bqLcFwbMRcp1cpQGv/HBuvYH8w0BjjBReP3GU2hQG0PIN1rVrTCJfQNz3U3V9v0a7b+vQkJC1A7twMzyDJtHRkZthID3NiRAPCZjyd8AXNpOQPH4Lh3nKy6Df0TbK+0PJ3vi9HzVwOdU3+cWNXc0s7xR2Jm0g3QpNQ68r48p+fYOUbMTOr M+zhVrVE 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Wed, Nov 11, 2026 at 06:13:18PM +0000, Lorenzo Stoakes (ARM) wrote: > From: Josef Bacik > > A THP that device migration maps into an empty PMD can end up on the wrong > deferred split list when it is later queued for a deferred split. > > This occurs if the memcg has no heads yet (resulting in a warning if > CONFIG_DEBUG_VM is set), where deferred_split_folio() will then queue the > folio on the parent's list. > > Once a later THP fault in the same memcg allocates the heads, > __folio_unqueue_deferred_split() and the split path resolve folio_memcg() > to the memcg's own, now existing, list and take its lock. They then > list_del_init() a folio that is linked on the parent's list, racing with > anything holding the parent's lock, and both lists' nr_items counts go > wrong. > > Resolve the issue by calling folio_memcg_alloc_deferred() after the charge > in migrate_vma_insert_huge_pmd_page() and falling back like a failed charge > if that fails, as __do_huge_pmd_anonymous_page() does. > > This was originally discovered by code inspection. > > Fixes: fafaeceb89a5 ("mm: switch deferred split shrinker to list_lru") > Cc: > Signed-off-by: Josef Bacik > [ ljs: tweak commit message, remove unnecessary comment ] > Signed-off-by: Lorenzo Stoakes (ARM) > --- > mm/migrate_device.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/mm/migrate_device.c b/mm/migrate_device.c > index 9e19fadfa65c..9e5dab8e1b77 100644 > --- a/mm/migrate_device.c > +++ b/mm/migrate_device.c > @@ -833,6 +833,13 @@ static int migrate_vma_insert_huge_pmd_page(struct migrate_vma *migrate, > goto abort; > } > > + if (folio_memcg_alloc_deferred(folio)) { > + count_vm_event(THP_FAULT_FALLBACK); > + count_mthp_stat(HPAGE_PMD_ORDER, MTHP_STAT_ANON_FAULT_FALLBACK); > + ret = -ENOMEM; > + goto abort; > + } > + > __folio_mark_uptodate(folio); > > pgtable = pte_alloc_one(vma->vm_mm); > Could you also clarify, if it is just the nr_items accounting that is a concern? Acked-by: Balbir Singh