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Commit65edfda6f3("mm/rmap: extend rmap and migration support device-private entries") updated set_pmd_migration_entry() to use pmdp_huge_get_and_clear() in the softleaf case, but made no further adjustments to the function itself. Therefore this function continues to incorrectly use pmd_write(), pmd_soft_dirty() and pmd_uffd_wp() to determine whether the installed migration entry should be marked writable, softdirty or uffd-wp respectively. Whilst all are incorrect, the most problematic of these is pmd_write(), as this can lead to corrupted rmap state. On x86-64 _PAGE_SWP_SOFT_DIRTY is aliased to _PAGE_RW. So calling pmd_write() on a softleaf will return the softdirty state encoded in the entry, assuming CONFIG_MEM_SOFT_DIRTY was enabled. This was observed when running the hmm.hmm_device_private.anon_write_child selftest: 1. The test faults in a range then migrates it such that a device-private THP range is established. 2. The parent then migrates it to a device-private writable PMD entry whose folio is entirely AnonExclusive with entire_mapcount=1, softdirty set (accidentally correct write state). 3. The parent forks and the PMD entries are set to device-private read only entries, entire_mapcount=2, softdirty still set. 4. [BUG] The child writes to the range then migrates to RAM - intending to install non-writable migration entries - but replacing parent and child PMD mappings with WRITABLE entries due to misinterpreting the softdirty bit. 5. In remove_migration_pmd(), if !softleaf_is_migration_read(entry) we set the RMAP_EXCLUSIVE flag when calling folio_add_anon_rmap_pmd() for both parent and child, which are therefore AnonExclusive. 6. [SPLAT] Child sets migrated folio entire_mapcount=1, parent sets entire_mapcount=2 and we end up with an AnonExclusive folio with entire_mapcount=2! Assert fires in __folio_add_anon_rmap(): VM_WARN_ON_FOLIO(folio_test_large(folio) && folio_entire_mapcount(folio) > 1 && PageAnonExclusive(cur_page), folio) This patch fixes the issue by correctly referencing the softleaf entry fields for writable, softdirty and uffd-wp in set_pmd_migration_entry(). It also only updates A/D flags if the entry is present as these are otherwise not meaningful for a softleaf entry. This patch also flips the if (!present) { ... } else { ... } logic in set_pmd_migration_entry() so it is easier to understand, and adds some comments to make things clearer. I was able to bisect this to commit775465fd26("lib/test_hmm: add zone device private THP test infrastructure") which first exposes this bug as it was the commit that permitted test_hmm to generate the test. However commit65edfda6f3("mm/rmap: extend rmap and migration support device-private entries") is the commit that actually enabled this behaviour. Link: https://lore.kernel.org/20260601083044.57132-1-ljs@kernel.org Fixes:65edfda6f3("mm/rmap: extend rmap and migration support device-private entries") Signed-off-by: Lorenzo Stoakes <ljs@kernel.org> Acked-by: David Hildenbrand (Arm) <david@kernel.org> Reviewed-by: Dev Jain <dev.jain@arm.com> Reviewed-by: Balbir Singh <balbirs@nvidia.com> Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com> Reviewed-by: Oscar Salvador (SUSE) <osalvador@kernel.org> Reviewed-by: Barry Song <baohua@kernel.org> Reviewed-by: Lance Yang <lance.yang@linux.dev> Reviewed-by: Zi Yan <ziy@nvidia.com> Cc: Baolin Wang <baolin.wang@linux.alibaba.com> Cc: Liam R. Howlett <liam@infradead.org> Cc: Nico Pache <npache@redhat.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Cc: SeongJae Park <sj@kernel.org> Cc: Wei Yang <richard.weiyang@gmail.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
5116 lines
140 KiB
5116 lines
140 KiB