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

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/[email protected]
Fixes: d245f9b ("mm/zone_device: support large zone device private folios")
Signed-off-by: John Groves <[email protected]>
Acked-by: David Hildenbrand (Red Hat) <[email protected]>
Reviewed-by: Dan Williams <[email protected]>
Tested-by: Alison Schofield <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: "Darrick J. Wong" <[email protected]>
Cc: Gregory Price <[email protected]>
Cc: Oscar Salvador <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: jagalactic

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));
 	}
 
 	/*