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It takes some effort to get XFS to do a PMD fault, but > > instructions to reproduce it are below. > > > > The VM_WARN_ON_ONCE(folio_test_large(folio)) check in > > free_zone_device_folio() incorrectly triggers for MEMORY_DEVICE_FS_DAX > > when PMD (2MB) mappings are used. > > > > FS-DAX legitimately creates large file-backed folios when handling PMD > > faults. This is a core feature of FS-DAX that provides significant > > performance benefits by mapping 2MB regions directly to persistent > > memory. When these mappings are unmapped, the large folios are freed > > through free_zone_device_folio(), which triggers the spurious warning. > > > > The warning was introduced by commit that added support for large zone > > device private folios. However, that commit did not account for FS-DAX > > file-backed folios, which have always supported large (PMD-sized) > > mappings. > > Oh, I was not copied on: > > d245f9b4ab80 mm/zone_device: support large zone device private folios > > ...I should probably add myself as a reviewer to the MEMORY HOT(UN)PLUG > entry in MAINTAINERS at least for the mm/mememap.c bits. > > Now, why is the warning there in the first place? Lets wait for Balbir to comment but I suspect it's just a mistake in d245f9b4ab80 ("mm/zone_device: support large zone device private folios"): - /* - * Note: we don't expect anonymous compound pages yet. Once supported - * and we could PTE-map them similar to THP, we'd have to clear - * PG_anon_exclusive on all tail pages. - */ if (folio_test_anon(folio)) { - VM_BUG_ON_FOLIO(folio_test_large(folio), folio); - __ClearPageAnonExclusive(folio_page(folio, 0)); + for (i = 0; i < nr; i++) + __ClearPageAnonExclusive(folio_page(folio, i)); + } else { + VM_WARN_ON_ONCE(folio_test_large(folio)); } The warning never applied to !folio_test_anon() folios and was just a reminder for the comment above which was deleted because it was fixed. Therefore the warning should also have been deleted. > I.e. what is the risk of just doing this fixup: None, I think that is the correct fix. > diff --git a/mm/memremap.c b/mm/memremap.c > index 4c2e0d68eb27..63c6ab4fdf08 100644 > --- a/mm/memremap.c > +++ b/mm/memremap.c > @@ -427,8 +427,6 @@ void free_zone_device_folio(struct folio *folio) > if (folio_test_anon(folio)) { > for (i = 0; i < nr; i++) > __ClearPageAnonExclusive(folio_page(folio, i)); > - } else { > - VM_WARN_ON_ONCE(folio_test_large(folio)); > } > > /* >