Re: [PATCH v2] mm: fault in complete folios instead of individual pages for tmpfs

[Lorenzo Stoakes]

On Fri, Jul 04, 2025 at 11:19:26AM +0800, Baolin Wang wrote:
> After commit acd7ccb284b8 ("mm: shmem: add large folio support for tmpfs"),
> tmpfs can also support large folio allocation (not just PMD-sized large
> folios).
>
> However, when accessing tmpfs via mmap(), although tmpfs supports large folios,
> we still establish mappings at the base page granularity, which is unreasonable.
>
> We can map multiple consecutive pages of a tmpfs folios at once according to
> the size of the large folio. On one hand, this can reduce the overhead of page
> faults; on the other hand, it can leverage hardware architecture optimizations
> to reduce TLB misses, such as contiguous PTEs on the ARM architecture.
>
> Moreover, tmpfs mount will use the 'huge=' option to control large folio
> allocation explicitly. So it can be understood that the process's RSS statistics
> might increase, and I think this will not cause any obvious effects for users.
>
> Performance test:
> I created a 1G tmpfs file, populated with 64K large folios, and write-accessed it
> sequentially via mmap(). I observed a significant performance improvement:
>
> Before the patch:
> real	0m0.158s
> user	0m0.008s
> sys	0m0.150s
>
> After the patch:
> real	0m0.021s
> user	0m0.004s
> sys	0m0.017s

Wow!

>
> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>

Seems reasonable, if we explicitly support larger folios in tmpfs now as
well as anon shmem (what a concept...)

So,

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
> Changes from v1:
>  - Drop the unnecessary IS_ALIGNED() check, per David.
>  - Update the commit message, per David.
> ---
>  mm/memory.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 0f9b32a20e5b..9944380e947d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5383,10 +5383,10 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>
>  	/*
>  	 * Using per-page fault to maintain the uffd semantics, and same
> -	 * approach also applies to non-anonymous-shmem faults to avoid
> +	 * approach also applies to non shmem/tmpfs faults to avoid
>  	 * inflating the RSS of the process.
>  	 */
> -	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
> +	if (!vma_is_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
>  	    unlikely(needs_fallback)) {
>  		nr_pages = 1;
>  	} else if (nr_pages > 1) {
> --
> 2.43.5
>