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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 <john@groves.net> Acked-by: David Hildenbrand (Red Hat) <david@kernel.org> Reviewed-by: Dan Williams <dan.j.williams@intel.com> Tested-by: Alison Schofield <alison.schofield@intel.com> Cc: Alistair Popple <apopple@nvidia.com> Cc: Balbir Singh <bsingharora@gmail.com> Cc: "Darrick J. Wong" <djwong@kernel.org> Cc: Gregory Price <gourry@gourry.net> Cc: Oscar Salvador <osalvador@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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