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

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Message-Id: <20250704151858.73d35a24b4c2f53bdb0c1b85@linux-foundation.org>
Date: Fri, 4 Jul 2025 15:18:58 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: hughd@...gle.com, david@...hat.com, ziy@...dia.com,
 lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, npache@...hat.com,
 ryan.roberts@....com, dev.jain@....com, baohua@...nel.org, vbabka@...e.cz,
 rppt@...nel.org, surenb@...gle.com, mhocko@...e.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: fault in complete folios instead of individual
 pages for tmpfs

On Fri,  4 Jul 2025 11:19:26 +0800 Baolin Wang <baolin.wang@...ux.alibaba.com> 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:

That doesn't sound like a crazy thing to do.

> Before the patch:
> real	0m0.158s
> user	0m0.008s
> sys	0m0.150s
> 
> After the patch:
> real	0m0.021s
> user	0m0.004s
> sys	0m0.017s

And look at that.

> 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) {

and that's it?

I'm itching to get this into -stable, really.  What LTS user wouldn't
want this?  Could it be viewed as correcting an oversight in
acd7ccb284b8?