[PATCH] mm: shmem: fix the shmem large folio allocation for the i915 driver

[Hugh Dickins]

On Mon, 28 Jul 2025, Baolin Wang wrote:

> After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"),
> we extend the 'huge=' option to allow any sized large folios for tmpfs,
> which means tmpfs will allow getting a highest order hint based on the size
> of write() and fallocate() paths, and then will try each allowable large order.
> 
> However, when the i915 driver allocates shmem memory, it doesn't provide hint
> information about the size of the large folio to be allocated, resulting in
> the inability to allocate PMD-sized shmem, which in turn affects GPU performance.
> 
> To fix this issue, add the 'end' information for shmem_read_folio_gfp()  to help
> allocate PMD-sized large folios. Additionally, use the maximum allocation chunk
> (via mapping_max_folio_size()) to determine the size of the large folios to
> allocate in the i915 driver.
> 
> Fixes: acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs")
> Reported-by: Patryk Kowalczyk <patryk at kowalczyk.ws>
> Reported-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Tested-by: Patryk Kowalczyk <patryk at kowalczyk.ws>
> Signed-off-by: Baolin Wang <baolin.wang at linux.alibaba.com>
> ---
>  drivers/gpu/drm/drm_gem.c                 | 2 +-
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c | 7 ++++++-
>  drivers/gpu/drm/ttm/ttm_backup.c          | 2 +-
>  include/linux/shmem_fs.h                  | 4 ++--
>  mm/shmem.c                                | 7 ++++---
>  5 files changed, 14 insertions(+), 8 deletions(-)

I know I said "I shall not object to a temporary workaround to suit the
i915 driver", but really, I have to question this patch.  Why should any
change be required at the drivers/gpu/drm end?

And in drivers/gpu/drm/{i915,v3d} I find they are using huge=within_size:
I had been complaining about the userspace regression in huge=always,
and thought it had been changed to behave like huge=within_size,
but apparently huge=within_size has itself regressed too.

Please explain why the below is not a better patch for i915 and v3d
(but still a temporary workaround, because the root of the within_size
regression must lie deeper, in the handling of write_end versus i_size).

Hugh

---
 mm/shmem.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 3a5a65b1f41a..c67dfc17a819 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -5928,8 +5928,8 @@ struct folio *shmem_read_folio_gfp(struct address_space *mapping,
 	struct folio *folio;
 	int error;
 
-	error = shmem_get_folio_gfp(inode, index, 0, &folio, SGP_CACHE,
-				    gfp, NULL, NULL);
+	error = shmem_get_folio_gfp(inode, index, i_size_read(inode),
+				    &folio, SGP_CACHE, gfp, NULL, NULL);
 	if (error)
 		return ERR_PTR(error);