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 sUE/b/Y5 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Jun 16, 2026 at 03:07:53PM +0200, David Hildenbrand (Arm) wrote: > On 6/16/26 14:30, Lance Yang wrote: > > > > On Tue, Jun 16, 2026 at 06:34:36AM +0000, Wei Yang wrote: > > [...] > >> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > >> index 2ccbabfb2cc1..21635fab209c 100644 > >> --- a/mm/page_vma_mapped.c > >> +++ b/mm/page_vma_mapped.c > >> @@ -243,40 +243,28 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > >> */ > >> pmde = pmdp_get_lockless(pvmw->pmd); > >> > >> - if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) { > >> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); > >> - pmde = *pvmw->pmd; > >> - if (!pmd_present(pmde)) { > >> - softleaf_t entry; > >> - > >> - if (!thp_migration_supported() || > >> - !(pvmw->flags & PVMW_MIGRATION)) > >> - return not_found(pvmw); > >> - entry = softleaf_from_pmd(pmde); > >> - > >> - if (!softleaf_is_migration(entry) || > >> - !check_pmd(softleaf_to_pfn(entry), pvmw)) > >> - return not_found(pvmw); > >> - return true; > >> - } > >> - if (likely(pmd_trans_huge(pmde))) { > >> - if (pvmw->flags & PVMW_MIGRATION) > >> - return not_found(pvmw); > >> - if (!check_pmd(pmd_pfn(pmde), pvmw)) > >> - return not_found(pvmw); > >> - return true; > >> - } > >> - /* 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); > >> - > >> - if (softleaf_is_device_private(entry)) { > >> - pvmw->ptl = pmd_lock(mm, pvmw->pmd); > >> - return true; > >> - } > >> + if (pmd_present(pmde)) { > >> + if (!pmd_leaf(pmde)) > >> + goto pte_table; > >> + if (pvmw->flags & PVMW_MIGRATION) > >> + return not_found(pvmw); > >> + if (!check_pmd(pmd_pfn(pmde), pvmw)) > >> + return not_found(pvmw); > >> + } else if (pmd_is_migration_entry(pmde)) { > >> + softleaf_t entry = softleaf_from_pmd(pmde); > >> + > >> + if (!(pvmw->flags & PVMW_MIGRATION)) > >> + return not_found(pvmw); > > > > Looked at history a bit, and I wonder if this changed something old > > here ... > > > > Since 616b8371539a ("mm: thp: enable thp migration in generic path"), PMD > > migration handling took PTL before doing PVMW_MIGRATION/PFN checks, > > including not_found() cases. So lockless PMD read was just a filter ... > > > > With this fix, true case gets final pmd_same() check, but this > > not_found() case happens before taking PTL. > > > > So a !PVMW_MIGRATION walker could race with someone, e.g. > > remove_migration_pmd(): we make the not_found() decision from old PMD > > value that still says "migration", while real *pvmw->pmd may already be > > present again. We return without ever taking PTL :) > > > > Not sure about practical fallout, but should these PMD-level not_found() > > cases also take PTL and restart if PMD changed? > I was hoping that we could so something similar to the PTE case. > > In map_pte(), we check whether the PMD changed, which is slightly different. > > The actual check happens in check_pte() after grabbing the PTL. > > For the case you describe, map_pte() would find !pte_none(ptent) ... > !pte_present(ptent) and !is_migration, and effectively grab the lock and proceed > to check_pte(). > > In check_pte() we re-check under lock indeed. > Thinking of the practical fallout, not finding the PMD for a non migration worker should be OK. Is there a case where it's not OK to report the old state. Balbir