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As a result, when page_vma_mapped_walk() encounters device-private PMD entries, it takes no action other than to acquire the PMD lock and exit. However this is highly problematic for two reasons - firstly, device private entries possess a PFN so check_pmd() needs to be called to ensure an overlapping PFN range. Secondly, and more importantly, if PVMW_MIGRATION is set the caller assumes the returned entry is a migration entry, resulting in memory corruption when the caller tries to interpret the device private entry as such. In addition, commit 146287290023 ("mm/huge_memory: implement device-private THP splitting") allowed device private PMDs to be split like THP mappings, but again did not update this code path. As a result, we might race a PMD split prior to acquiring the PMD lock. This patch addresses all of these issues by invoking check_pmd(), ensuring PMVW_MIGRATION is not set and checks whether a split raced us we do for PMD THP and migration entries. Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") Cc: Signed-off-by: Wei Yang Suggested-by: David Hildenbrand Cc: David Hildenbrand Cc: Balbir Singh Cc: SeongJae Park Cc: Zi Yan Cc: Lorenzo Stoakes Cc: Lance Yang --- v4: * refine subject and commit log based on Lorenzo's suggestion * put pmd device-private entry handling in its own if branch, suggested by Lorenzo v3: * remove cleanup part, only fix the issue for device-private entry * refine user effect description based on Lorenzo's suggestion v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@gmail.com/T/#u * specify the possible error case of current code and user visible effect * besides fix, cleanup the pmd entry handling based on David's suggestion v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@gmail.com/ --- mm/page_vma_mapped.c | 20 +++++++++++++++----- 1 file changed, 15 insertions(+), 5 deletions(-) diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index 2ccbabfb2cc1..17dff8aab9f9 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -269,14 +269,24 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) /* THP pmd was split under us: handle on pte level */ spin_unlock(pvmw->ptl); pvmw->ptl = NULL; - } else if (!pmd_present(pmde)) { - const softleaf_t entry = softleaf_from_pmd(pmde); + } else if (pmd_is_device_private_entry(pmde)) { + softleaf_t entry; + + pvmw->ptl = pmd_lock(mm, pvmw->pmd); + pmde = *pvmw->pmd; + entry = softleaf_from_pmd(pmde); - if (softleaf_is_device_private(entry)) { - pvmw->ptl = pmd_lock(mm, pvmw->pmd); + if (likely(softleaf_is_device_private(entry))) { + if (pvmw->flags & PVMW_MIGRATION) + return not_found(pvmw); + if (!check_pmd(softleaf_to_pfn(entry), pvmw)) + return not_found(pvmw); return true; } - + /* device-private pmd was split under us: handle on pte level */ + spin_unlock(pvmw->ptl); + pvmw->ptl = NULL; + } else if (!pmd_present(pmde)) { if ((pvmw->flags & PVMW_SYNC) && thp_vma_suitable_order(vma, pvmw->address, PMD_ORDER) && -- 2.34.1