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

[<dan.j.williams@intel.com>]

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.

Oh, I was not copied on:

d245f9b4ab80 mm/zone_device: support large zone device private folios

...I should probably add myself as a reviewer to the MEMORY HOT(UN)PLUG
entry in MAINTAINERS at least for the mm/mememap.c bits.

Now, why is the warning there in the first place?

I.e. what is the risk of just doing this fixup:

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));
        }
 
        /*