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

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

[John Groves]

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.

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@groves.net>
---

Change since V1: Deleted the warning altogether, rather than exempting
fs-dax.

=== How to reproduce ===

A reproducer is available at:

    git clone https://github.com/jagalactic/dax-pmd-test.git
    cd xfs-dax-test
    make
    sudo make test

This will set up XFS on pmem with 2MB stripe alignment and run a test
that triggers the warning.

Alternatively, follow the manual steps below.

Prerequisites:
  - Linux kernel with FS-DAX support and CONFIG_DEBUG_VM=y
  - A pmem device (real or emulated)
  - An fsdax namespace configured via ndctl as /dev/pmem0

Manual steps:

1. Create an fsdax namespace (if not already present):
   # ndctl create-namespace -m fsdax -e namespace0.0

2. Create XFS with 2MB stripe alignment:
   # mkfs.xfs -f -d su=2m,sw=1 /dev/pmem0
   # mount -o dax /dev/pmem0 /mnt/pmem

3. Compile and run the reproducer:
   # gcc -Wall -O2 -o dax_pmd_test dax_pmd_test.c
   # ./dax_pmd_test /mnt/pmem/testfile

4. Check dmesg for the warning:
   WARNING: mm/memremap.c:431 at free_zone_device_folio+0x.../0x...

Note: The 2MB stripe alignment (-d su=2m,sw=1) is critical. XFS normally
allocates blocks at arbitrary offsets, causing PMD faults to fall back
to PTE faults. The stripe alignment forces 2MB-aligned allocations,
allowing PMD faults to succeed and exposing this bug.


 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));
 	}
 
 	/*

base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
-- 
2.49.0

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

[David Hildenbrand (Red Hat)]

On 12/19/25 13:37, John Groves 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.
> 
> 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@groves.net>
> ---
> 
> Change since V1: Deleted the warning altogether, rather than exempting
> fs-dax.
> 
> === How to reproduce ===
> 
> A reproducer is available at:
> 
>      git clone https://github.com/jagalactic/dax-pmd-test.git
>      cd xfs-dax-test
>      make
>      sudo make test
> 
> This will set up XFS on pmem with 2MB stripe alignment and run a test
> that triggers the warning.
> 
> Alternatively, follow the manual steps below.
> 
> Prerequisites:
>    - Linux kernel with FS-DAX support and CONFIG_DEBUG_VM=y
>    - A pmem device (real or emulated)
>    - An fsdax namespace configured via ndctl as /dev/pmem0
> 
> Manual steps:
> 
> 1. Create an fsdax namespace (if not already present):
>     # ndctl create-namespace -m fsdax -e namespace0.0
> 
> 2. Create XFS with 2MB stripe alignment:
>     # mkfs.xfs -f -d su=2m,sw=1 /dev/pmem0
>     # mount -o dax /dev/pmem0 /mnt/pmem
> 
> 3. Compile and run the reproducer:
>     # gcc -Wall -O2 -o dax_pmd_test dax_pmd_test.c
>     # ./dax_pmd_test /mnt/pmem/testfile
> 
> 4. Check dmesg for the warning:
>     WARNING: mm/memremap.c:431 at free_zone_device_folio+0x.../0x...
> 
> Note: The 2MB stripe alignment (-d su=2m,sw=1) is critical. XFS normally
> allocates blocks at arbitrary offsets, causing PMD faults to fall back
> to PTE faults. The stripe alignment forces 2MB-aligned allocations,
> allowing PMD faults to succeed and exposing this bug.
> 
> 
>   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));
>   	}
>   

LGTM

Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>

-- 
Cheers

David

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

[dan.j.williams]

John Groves wrote:
[..]
> 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.

As you note, this patch no longer exempts MEMORY_DEVICE_FS_DAX explicitly, it just
removes the bogus warning, so maybe Andrew can adjust this note on
applying?

> Fixes: d245f9b4ab80 ("mm/zone_device: support large zone device private folios")
> Signed-off-by: John Groves <john@groves.net>
> ---
> 
> Change since V1: Deleted the warning altogether, rather than exempting
> fs-dax.
> 
> === How to reproduce ===
> 
> A reproducer is available at:
> 
>     git clone https://github.com/jagalactic/dax-pmd-test.git
>     cd xfs-dax-test
>     make
>     sudo make test

Thanks John, outside of the fixup above, this looks good to me.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>

Now, my first thoughts when seeing this were:

"ooh, I want that test in the regression suite"

...then:

"wait, that sounds exactly like the existing dax.sh test [1]"

[1]: https://github.com/pmem/ndctl/blob/main/test/dax.sh

Alison reports that indeed that existing test triggers the problem which
indicates some process problems to solve.

- Folks touching mm/memremap.c (and anything dax related) do not know
  about / run the regression tests.

- The bespoke nature of the dax testing environment needs some work to
  get it into a kselftest amenable flow, or otherwise need more
  automation to run those tests automatically upon seeing those files
  touched in linux-next so folks see breakage like this earlier.

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@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.
> 
> 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@groves.net>
> ---

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