Re: [PATCH V2] mm/memremap: fix spurious large folio warning for FS-DAX

[Alison Schofield]

On Fri, Dec 19, 2025 at 06:37:17AM -0600, John Groves wrote:
> From: John Groves <John@xxxxxxxxxx>
> 
> 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.
> 
> Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
> Signed-off-by: John Groves <john@xxxxxxxxxx>
> ---

Tested-by: Alison Schofield <alison.schofield@xxxxxxxxx>