From 077d925b60c320027dd64b69e0ab2dd2e00ed45c Mon Sep 17 00:00:00 2001 From: John Groves Date: Fri, 19 Dec 2025 06:37:17 -0600 Subject: [PATCH] mm/memremap: fix spurious large folio warning for FS-DAX This patch addresses a warning that I discovered while working on famfs, which is an fs-dax file system that virtually always does PMD faults (next famfs patch series coming after the holidays). However, XFS also does PMD faults in fs-dax mode, and it also triggers the warning. 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. The check distinguishes between anonymous folios (which clear AnonExclusive flags for each sub-page) and file-backed folios. For file-backed folios, it assumes large folios are unexpected - but this assumption is incorrect for FS-DAX. The fix is to exempt MEMORY_DEVICE_FS_DAX from the large folio warning, allowing FS-DAX to continue using PMD mappings without triggering false warnings. Link: https://lkml.kernel.org/r/20251219123717.39330-1-john@groves.net Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios") Signed-off-by: John Groves Acked-by: David Hildenbrand (Red Hat) Reviewed-by: Dan Williams Tested-by: Alison Schofield Cc: Alistair Popple Cc: Balbir Singh Cc: "Darrick J. Wong" Cc: Gregory Price Cc: Oscar Salvador Signed-off-by: Andrew Morton --- mm/memremap.c | 2 -- 1 file changed, 2 deletions(-) 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)); } /*