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

[Lance Yang]

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@xxxxxxxxxxxxxxxxx/
>Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
>---
>Note: for more investigation and test data, see:
>https://lore.kernel.org/all/116df9f9-4db7-40d4-a4a4-30a87c0feffa@xxxxxxxxxxxxxxxxx/
>Thanks Kefeng for confirming the performance issue.
>---

Thanks for tackling this!

Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>