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

[Zi Yan <ziy@nvidia.com>]

On 22 Apr 2026, at 21:41, 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>
> ---
> 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(-)
>
Thank you for the investigation and explanation.

Acked-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi