Re: [PATCH v8 14/15] filemap: Add support for authoritative mappings

On 7/31/2026 1:07 PM, Matthew Wilcox (Oracle) wrote:
> An authoritative mapping knows about all the folios in the mapping. If=20
> read() finds a missing folio, there's no reason to allocate one and try=20
> to read it because we know it's a zero region of the file. We can just=20
> call iov_iter_zero() instead.=E2=80=8A
>=20
>=20
> 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.
>=20
> Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
> ---
>   include/linux/pagemap.h | 11 +++++++++++
>   mm/filemap.c            | 18 ++++++++++++++++++
>   2 files changed, 29 insertions(+)
>=20
> 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 =3D 9,
>   	AS_KERNEL_FILE =3D 10,	/* mapping for a fake kernel file that shouldn't
>   				   account usage to user cgroups */
> +	AS_AUTHORITATIVE =3D 11,	/* If we miss in the page cache, it's a hole */
>   	/* Bits 16-25 are used for FOLIO_ORDER */
>   	AS_FOLIO_ORDER_BITS =3D 5,
>   	AS_FOLIO_ORDER_MIN =3D 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);
>   }
>  =20
> +static inline void mapping_set_authoritative(struct address_space *mappi=
ng)
> +{
> +	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 *mappin=
g)
>   {
>   	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, si=
ze_t count,
>   	if (!folio_batch_count(fbatch)) {
>   		DEFINE_READAHEAD(ractl, filp, &filp->f_ra, mapping, index);
>  =20
> +		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 io=
v_iter *iter,
>   			goto put_folios;
>   		end_offset =3D min_t(loff_t, isize, iocb->ki_pos + iter->count);
>  =20
> +		if (!folio_batch_count(&fbatch)) {
> +			size_t fsize =3D mapping_min_folio_nrbytes(mapping);
> +			size_t offset =3D iocb->ki_pos & (fsize - 1);
> +			size_t bytes =3D min_t(loff_t, end_offset - iocb->ki_pos,
> +					     fsize - offset);
> +			size_t copied =3D iov_iter_zero(bytes, iter);
> +
> +			already_read +=3D copied;
> +			iocb->ki_pos +=3D copied;
> +			last_pos =3D iocb->ki_pos;
> +
> +			if (copied < bytes)
> +				error =3D -EFAULT;
> +			continue;
> +		}
> +
>   		/*
>   		 * Once we start copying data, we don't want to be touching any
>   		 * cachelines that might be contended:
> --=20
> 2.47.3
>=20

Very nice change.

Reviewed-by: Jane Chu <[email protected]>

thanks,
-jane