Re: [linux-next:master] [mm] 7b32f64bc5: pts.svt-av1.Preset13.Bosphorus4K.frames_per_second 45.8% regression

hi, Bo,

On Thu, Jul 23, 2026 at 10:45:27AM +0800, Bo Zhang wrote:
> From: zhangbo56 <[email protected]>
> 
> On Thu 18-06-26 16:00:42, kernel test robot wrote:
> > kernel test robot noticed a 45.8% regression of
> > pts.svt-av1.Preset13.Bosphorus4K.frames_per_second on:
> >
> > commit: 7b32f64bc512b40b268776c5ac4d354b325b3197
> > ("mm: limit filemap_fault readahead to VMA boundaries")
> >
> >     169.95 +/- 6%     -45.8%      92.15 +/- 10%  pts.svt-av1.Preset13.Bosphorus4K.frames_per_second
> >     220.57 +/- 3%    +870.9%       2141        pts.time.major_page_faults
> 
> Hi Oliver,
> 
> Could you help test if the below patch fixes the regression?

quite sorry that we lost the machine we reported this regression, and since
pts running need some specific settings in our test framework, we still need
some time to setup on other test machines, we cannot start the test soon. will
keep you updated.

thanks for your paticence and sorry for any inconvenience.


> 
> The 870% increase in major faults suggests that readahead is being cut
> short too aggressively. The current approach unconditionally sets
> _max_index on every fault, which likely prevents readahead from
> prefetching ahead effectively for sequential access patterns.
> 
> The fix: only limit readahead when the fault is close to the VMA end --
> if there are fewer pages remaining in the VMA than ra_pages, set
> _max_index. Otherwise, do nothing.
> 
> For a 4MB VMA with ra_pages=32, only faults in the last 32 pages (128KB)
> trigger the limit. The other 99.9% of faults see no change at all.
> 
> Similarly, only clamp the read-around start when the fault is near the
> VMA beginning.
> 
> This applies on top of 7b32f64bc512 and can be applied with git am.
> 
> Reported-by: kernel test robot <[email protected]>
> Signed-off-by: Bo Zhang <[email protected]>
> ---
>  mm/filemap.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 97772a05a18e..9f1e1c9ea6df 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3313,8 +3313,11 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  	vm_flags_t vm_flags = vmf->vma->vm_flags;
>  	bool force_thp_readahead = false;
>  	unsigned short mmap_miss;
> +	unsigned long vma_end_pgoff = vmf->vma->vm_pgoff + vma_pages(vmf->vma);
> +	unsigned long vma_pages_left = vma_end_pgoff - vmf->pgoff;
>  
> -	ractl._max_index = vmf->vma->vm_pgoff + vma_pages(vmf->vma) - 1;
> +	if (vma_pages_left < ra->ra_pages)
> +		ractl._max_index = vma_end_pgoff - 1;
>  
>  	/* Use the readahead code, even if readahead is disabled */
>  	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> @@ -3398,7 +3401,8 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
>  		 * mmap read-around
>  		 */
>  		ra->start = max_t(long, 0, vmf->pgoff - ra->ra_pages / 2);
> -		ra->start = max(ra->start, vmf->vma->vm_pgoff);
> +		if (vmf->pgoff - vmf->vma->vm_pgoff < ra->ra_pages / 2)
> +			ra->start = max(ra->start, vmf->vma->vm_pgoff);
>  		ra->size = ra->ra_pages;
>  		ra->async_size = ra->ra_pages / 4;
>  		ra->order = 0;
> @@ -3441,7 +3445,12 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>  	}
>  
>  	if (folio_test_readahead(folio)) {
> -		ractl._max_index = vmf->vma->vm_pgoff + vma_pages(vmf->vma) - 1;
> +		unsigned long vma_end_pgoff = vmf->vma->vm_pgoff +
> +					     vma_pages(vmf->vma);
> +		unsigned long vma_pages_left = vma_end_pgoff - vmf->pgoff;
> +
> +		if (vma_pages_left < ra->ra_pages)
> +			ractl._max_index = vma_end_pgoff - 1;
>  		fpin = maybe_unlock_mmap_for_io(vmf, fpin);
>  		page_cache_async_ra(&ractl, folio, ra->ra_pages);
>  	}
> -- 
> 2.34.1
>