Re: [PATCH v9 14/15] filemap: Add support for authoritative mappings

On Wed, Aug 05, 2026 at 10:05:54PM +0100, Matthew Wilcox (Oracle) wrote:
> An authoritative mapping knows about all the folios in the mapping.
> If read() finds a missing folio, there's no reason to allocate one and
> try to read it because we know it's a zero region of the file.  We can
> just call iov_iter_zero() instead.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
> Reviewed-by: Jane Chu <[email protected]>
> ---
>  include/linux/pagemap.h | 11 +++++++++++
>  mm/filemap.c            | 18 ++++++++++++++++++
>  2 files changed, 29 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 968b791cfd14..22d48935ffda 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -210,6 +210,7 @@ enum mapping_flags {
>  	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
>  	AS_KERNEL_FILE = 10,	/* mapping for a fake kernel file that shouldn't
>  				   account usage to user cgroups */
> +	AS_AUTHORITATIVE = 11,	/* If we miss in the page cache, it's a hole */
>  	/* Bits 16-25 are used for FOLIO_ORDER */
>  	AS_FOLIO_ORDER_BITS = 5,
>  	AS_FOLIO_ORDER_MIN = 16,
> @@ -345,6 +346,16 @@ static inline bool mapping_writeback_may_deadlock_on_reclaim(const struct addres
>  	return test_bit(AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM, &mapping->flags);
>  }
>  
> +static inline void mapping_set_authoritative(struct address_space *mapping)
> +{
> +	set_bit(AS_AUTHORITATIVE, &mapping->flags);
> +}
> +
> +static inline bool mapping_is_authoritative(const struct address_space *mapping)
> +{
> +	return test_bit(AS_AUTHORITATIVE, &mapping->flags);
> +}
> +
>  static inline gfp_t mapping_gfp_mask(const struct address_space *mapping)
>  {
>  	return mapping->gfp_mask;
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 26a5f18121f9..5a8cc20e624e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2697,6 +2697,8 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
>  	if (!folio_batch_count(fbatch)) {
>  		DEFINE_READAHEAD(ractl, filp, &filp->f_ra, mapping, index);
>  
> +		if (mapping_is_authoritative(mapping))
> +			return 0;
>  		if (iocb->ki_flags & IOCB_NOIO)
>  			return -EAGAIN;
>  		if (iocb->ki_flags & IOCB_NOWAIT)
> @@ -2853,6 +2855,22 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
>  			goto put_folios;
>  		end_offset = min_t(loff_t, isize, iocb->ki_pos + iter->count);
>  
> +		if (!folio_batch_count(&fbatch)) {
> +			size_t fsize = mapping_min_folio_nrbytes(mapping);
> +			size_t offset = iocb->ki_pos & (fsize - 1);
> +			size_t bytes = min_t(loff_t, end_offset - iocb->ki_pos,
> +					     fsize - offset);
> +			size_t copied = iov_iter_zero(bytes, iter);
> +
> +			already_read += copied;
> +			iocb->ki_pos += copied;
> +			last_pos = iocb->ki_pos;
> +
> +			if (copied < bytes)
> +				error = -EFAULT;
> +			continue;
> +		}
> +

I was wondering whether this could interact weirdly with EOF, but it doesn't
seem so.

Reviewed-by: Pedro Falcato <[email protected]>

-- 
Pedro