Re: [PATCH v3] mm: shmem: always support large folios for internal shmem mount

Kefeng Wang
2026-04-22T15:03:24Z

On 4/22/2026 2:28 PM, Baolin Wang wrote:
> CC Kefeng,
>
> On 4/21/26 9:39 PM, David Hildenbrand (Arm) wrote:
>> On 4/21/26 08:27, Baolin Wang wrote:
>>>
>>>
>>> On 4/21/26 3:00 AM, David Hildenbrand (Arm) wrote:
>>>> On 4/17/26 14:45, Baolin Wang wrote:
>>>>>
>>>>>
>>>>>
>>>>> Indeed. Good point.
>>>>>
>>>>>
>>>>> Not really. There could be files created before remount whose mappings
>>>>> don't support large folios (with 'huge=never' option), while files
>>>>> created after remount will have mappings that support large folios (if
>>>>> remounted with 'huge=always' option).
>>>>>
>>>>> It looks like the previous commit 5a90c155defa was also
>>>>> problematic. The
>>>>> huge mount option has introduced a lot of tricky issues:(
>>>>>
>>>>> Now I think Zi's previous suggestion should be able to clean up this
>>>>> mess? That is, calling mapping_set_large_folios() unconditionally for
>>>>> all shmem mounts, and revisiting Kefeng's first version to fix the
>>>>> performance issue.
>>>>
>>>> Okay, so you'll send a patch to just set mapping_set_large_folios()
>>>> unconditionally?
>>>
>>> I'm still hesitating on this. If we set mapping_set_large_folios()
>>> unconditionally, we need to re-fix the performance regression that was
>>> addressed by commit 5a90c155defa.
>>
>> Just so I can follow: where is the test for large folios that we would
>> unlock large folios and cause a regression?
>
> I spent some time investigating the performance regression that was
> addressed by commit 5a90c155defa ("tmpfs: don't enable large folios if
> not supported"). From my testing, I found that the performance issue no
> longer exists on upstream:
>
> mount tmpfs -t tmpfs -o size=50G /mnt/tmpfs
>
> 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.
>
> Later, I continued investigating and found that commit 665575cff098b
> ("filemap: move prefaulting out of hot write path") fixed the write
> operation performance.
>
> Base + revert 665575cff098b + revert 5a90c155defa:
> dd if=/dev/zero of=/mnt/tmpfs/test bs=400K count=10485 (3.0 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=800K count=5242 (2.9 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=1600K count=2621 (2.6 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=2200K count=1906 (2.6 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=3000K count=1398 (2.5 GB/s)
> dd if=/dev/zero of=/mnt/tmpfs/test bs=4500K count=932 (2.5 GB/s)
>
> We can see that after reverting commit 665575cff098b, there is a
> noticeable drop in write performance for tmpfs files.
>
> So my conclusion is that we can now safely revert commit 5a90c155defa to
> set mapping_set_large_folios() for all shmem mounts unconditionally.
>
> Kefeng, please correct me if I missed anything.

Hi Baolin，I found my testcases "bonnie Block/Re Write"

./bonnie -d /tmp -s Size (size is from 100,256,512,1024,2048,4096).

But the dd test is similar as well, and as commit 4e527d5841e2
("iomap: fault in smaller chunks for non-large folio mappings") said,
the issue is,

"If chunk is 2MB, total 512 pages need to be handled finally. During this
period, fault_in_iov_iter_readable() is called to check iov_iter readable
validity. Since only 4KB will be handled each time, below address space
will be checked over and over again"

But after 665575cff098b, fault_in_iov_iter_readable() is moved, so the
issue should be fixed.

+CC Dave，

Since 665575cff098b is works well in generic_perform_write(), I think we
could do the same optimization in iomap_write_iter()? but it seems
maintainer forget pickup them[1].

[1]