Re: [PATCH] Revert "tmpfs: don't enable large folios if not supported"

[Lorenzo Stoakes <ljs@kernel.org>]

On Thu, Apr 23, 2026 at 09:41:42AM +0800, Baolin Wang wrote:
> This reverts commit 5a90c155defa684f3a21f68c3f8e40c056e6114c.
>
> Currently, when shmem mounts are initialized, they only use 'sbinfo->huge' to
> determine whether the shmem mount supports large folios. However, for anonymous
> shmem, whether it supports large folios can be dynamically configured via sysfs
> interfaces, so setting or not setting mapping_set_large_folios() during initialization
> cannot accurately reflect whether anonymous shmem actually supports large folios,
> which has already caused some confusion[1].
>
> Moreover, for tmpfs mounts, relying on 'sbinfo->huge' cannot keep the mapping_set_large_folios()
> setting consistent across all mappings in the entire tmpfs mount. In other words,
> under the same tmpfs mount, after remount, we might end up with some mappings
> supporting large folios (calling mapping_set_large_folios()) while others don't.
>
> After some investigation, I found that the write performance regression addressed
> by commit 5a90c155defa has already been fixed by the following commit 665575cff098b
> ("filemap: move prefaulting out of hot write path"). See the following test data:
>
> Base:
> dd if=/dev/zero of=/mnt/tmpfs/test bs=400K count=10485 (3.2 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=800K count=5242 (3.2 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=1600K count=2621 (3.1 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=2200K count=1906 (3.0 GB/s )
> dd if=/dev/zero of=/mnt/tmpfs/test bs=3000K count=1398 (3.0 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=4500K count=932 (3.1 GB/s)
>
> Base + revert 5a90c155defa:
> dd if=/dev/zero of=/mnt/tmpfs/test bs=400K count=10485 (3.3 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=800K count=5242 (3.3 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=1600K count=2621 (3.2 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=2200K count=1906 (3.1 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/testbs=3000K count=1398 (3.0 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=4500K count=932 (3.1 GB/s)
>
> The data is basically consistent with minor fluctuation noise. So we can now
> safely revert commit 5a90c155defa to set mapping_set_large_folios() for all
> shmem mounts unconditionally.
>
> [1] https://lore.kernel.org/all/ec927492-4577-4192-8fad-85eb1bb43121@linux.alibaba.com/
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Acked-by: Lorenzo Stoakes <ljs@kernel.org>

As David asked, should we have a Fixes tag? What about cc: stable?

Cheers, Lorenzo

> ---
> Note: for more investigation and test data, see:
> https://lore.kernel.org/all/116df9f9-4db7-40d4-a4a4-30a87c0feffa@linux.alibaba.com/
> Thanks Kefeng for confirming the performance issue.
> ---
>  mm/shmem.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4ecefe02881d..dafbea53b22d 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3087,10 +3087,7 @@ static struct inode *__shmem_get_inode(struct mnt_idmap *idmap,
>  	cache_no_acl(inode);
>  	if (sbinfo->noswap)
>  		mapping_set_unevictable(inode->i_mapping);
> -
> -	/* Don't consider 'deny' for emergencies and 'force' for testing */
> -	if (sbinfo->huge)
> -		mapping_set_large_folios(inode->i_mapping);
> +	mapping_set_large_folios(inode->i_mapping);
>
>  	switch (mode & S_IFMT) {
>  	default:
> --
> 2.47.3
>