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

[John Groves <John@groves.net>]

On 25/12/18 04:03PM, Andrew Morton wrote:
> On Thu, 18 Dec 2025 09:58:02 +1100 Alistair Popple <apopple@nvidia.com> wrote:
> 
> > On 2025-12-18 at 08:13 +1100, John Groves <John@Groves.net> wrote...
> > > From: John Groves <John@Groves.net>
> > > 
> > > 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.
> > 
> > Yep, and I'm pretty sure devdax can also create large folios so we might need
> > a similar fix there. In fact looking at old vs. new code it seems we only ever
> > used to have this warning for anon folios, which I think could only ever be true
> > for DEVICE_PRIVATE or DEVICE_COHERENT folios.
> > 
> > So I suspect the proper fix is to just remove the warning entirely now that they
> > also support compound sizes.
> 
> So I'm assuming we can expect an updated version of this fix.

I'll send an update Friday morning

<snip>

Thanks Alistair, Dan and Andrew!

John