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

[John Groves]

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.

Signed-off-by: John Groves <john@xxxxxxxxxx>
---
=== 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.

=== Proposed fix ===

mm/memremap.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/memremap.c b/mm/memremap.c
index 4c2e0d68eb27..af37c3b4e39b 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -428,7 +428,12 @@ void free_zone_device_folio(struct folio *folio)
 		for (i = 0; i < nr; i++)
 			__ClearPageAnonExclusive(folio_page(folio, i));
 	} else {
-		VM_WARN_ON_ONCE(folio_test_large(folio));
+		/*
+		 * FS_DAX legitimately uses large file-mapped folios for
+		 * PMD mappings, so only warn for other device types.
+		 */
+		VM_WARN_ON_ONCE(pgmap->type != MEMORY_DEVICE_FS_DAX &&
+				folio_test_large(folio));
 	}
 
 	/*

base-commit: 8f0b4cce4481fb22653697cced8d0d04027cb1e8
-- 
2.49.0