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Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Joshua Hahn , Rakie Kim , Byungchul Park , Gregory Price , Ying Huang , Alistair Popple , Balbir Singh , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , =?UTF-8?q?Thomas=20Hellstr=C3=B6m?= , Francois Dugast , stable@vger.kernel.org Subject: [PATCH 2/4] mm/migrate_device: Apply the fault reference to the correct folio Date: Wed, 5 Aug 2026 04:33:36 -0700 Message-Id: <20260805113338.3742178-3-matthew.brost@intel.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260805113338.3742178-1-matthew.brost@intel.com> References: <20260805113338.3742178-1-matthew.brost@intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit __migrate_device_pages() computed extra_cnt once, from the head page of the source folio, and then passed the same value to folio_migrate_mapping() for every one of the @nr sub-folios produced by migrate_vma_split_unmapped_folio(). The extra reference the CPU fault holds only exists on the single folio that ends up containing the fault page. Claiming it for all of them makes folio_migrate_mapping() expect one reference too many on every other sub-folio, so it returns -EAGAIN and MIGRATE_PFN_MIGRATE is cleared for them. Only the fault page would migrate; the remaining HPAGE_PMD_NR - 1 pages would be restored to device memory, and the faulting access would immediately fault again. Compute extra_cnt per sub-folio instead, comparing against the source page for that entry. While at it, use the sub-folio's own mapping rather than the mapping that was read from the pre-split folio. This has been latent so far because the split it depends on could never succeed while the fault reference was held. Fixes: 4265d67e405a ("mm/migrate_device: add THP splitting during migration") Cc: Andrew Morton Cc: David Hildenbrand Cc: Lorenzo Stoakes Cc: Zi Yan Cc: Baolin Wang Cc: Liam R. Howlett Cc: Nico Pache Cc: Ryan Roberts Cc: Dev Jain Cc: Barry Song Cc: Lance Yang Cc: Usama Arif Cc: Joshua Hahn Cc: Rakie Kim Cc: Byungchul Park Cc: Gregory Price Cc: Ying Huang Cc: Alistair Popple Cc: Balbir Singh Cc: Maarten Lankhorst Cc: Maxime Ripard Cc: Thomas Zimmermann Cc: David Airlie Cc: Simona Vetter Cc: Thomas Hellström Cc: Francois Dugast Cc: dri-devel@lists.freedesktop.org Cc: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org Assisted-by: GitHub Copilot:claude-opus-5 Signed-off-by: Matthew Brost --- mm/migrate_device.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-) diff --git a/mm/migrate_device.c b/mm/migrate_device.c index d1d81bbbf795..65a6a15dba4d 100644 --- a/mm/migrate_device.c +++ b/mm/migrate_device.c @@ -1191,7 +1191,7 @@ static void __migrate_device_pages(unsigned long *src_pfns, struct page *page = migrate_pfn_to_page(src_pfns[i]); struct address_space *mapping; struct folio *newfolio, *folio; - int r, extra_cnt = 0; + int r; unsigned long nr = 1; if (!newpage) { @@ -1296,13 +1296,25 @@ static void __migrate_device_pages(unsigned long *src_pfns, BUG_ON(folio_test_writeback(folio)); - if (migrate && migrate->fault_page == page) - extra_cnt = 1; for (j = 0; j < nr && i + j < npages; j++) { - folio = page_folio(migrate_pfn_to_page(src_pfns[i+j])); + struct page *src_page = migrate_pfn_to_page(src_pfns[i+j]); + int extra_cnt = 0; + + folio = page_folio(src_page); newfolio = page_folio(migrate_pfn_to_page(dst_pfns[i+j])); - r = folio_migrate_mapping(mapping, newfolio, folio, extra_cnt); + /* + * The CPU fault holds an extra reference on the folio + * containing the fault page. @folio may have been + * split above, so the fault page only accounts for an + * extra reference on the folio it actually ended up + * in, not on every folio of the original THP. + */ + if (migrate && migrate->fault_page == src_page) + extra_cnt = 1; + + r = folio_migrate_mapping(folio_mapping(folio), newfolio, + folio, extra_cnt); if (r) src_pfns[i+j] &= ~MIGRATE_PFN_MIGRATE; else -- 2.34.1