Re: [PATCH] mm/filemap: fix page_cache_prev_miss() when no hole is found

[Vishal Moola <vishal.moola@gmail.com>]

On Sun, May 10, 2026 at 05:54:17PM -0400, Tal Zussman wrote:
> page_cache_prev_miss() is documented to return a value outside the
> searched range when no gap is found. However, the no-gap-found path
> returns xas.xa_index, which after a successful loop is the first index
> in the range. As such, that index is misreported as a gap.
> 
> The sole caller, page_cache_sync_ra(), uses the return value to estimate
> the cached run preceding a sequential read. In some cases, the buggy
> return value can undercount the contiguous range by one, shrinking the
> readahead window or pushing borderline requests into the
> small-random-read branch.
> 
> Mirror the fix in commit bbcaee20e03e ("readahead: fix return value of
> page_cache_next_miss() when no hole is found"): preserve max_scan in a
> separate variable across the loop and return `index - max_scan` from the
> no-gap-found path.

IMO, this way of fixing it hurts the readability. I'd prefer something
similar to the fix in the original commit. Or...

> -	while (max_scan--) {
> +	while (nr--) {
>  		void *entry = xas_prev(&xas);
>  		if (!entry || xa_is_value(entry))
> -			break;
> +			return xas.xa_index;
>  		if (xas.xa_index == ULONG_MAX)
> -			break;
> +			return ULONG_MAX;
>  	}

If I understand this correctly, couldn't we just do something like:
if (!max_scan)
	return xas.xa_index - 1;

> -	return xas.xa_index;
> +	return index - max_scan;
>  }
>  EXPORT_SYMBOL(page_cache_prev_miss);
>  
> 
> ---
> base-commit: e9dd96806dbc2d50a66770b6a86962bd5d601153
> change-id: 20260510-prev_miss_fix-fcb308472131
> 
> Best regards,
> -- 
> Tal Zussman <tz2294@columbia.edu>
>