Re: [PATCH] ceph: support RWF_DONTCACHE

[Viacheslav Dubeyko <vdubeyko@redhat.com>]

On Wed, 2026-04-01 at 07:31 +0200, Max Kellermann wrote:
> Similar to commit 902893e39076 ("NFS: Enable use of the RWF_DONTCACHE
> flag on the NFS client").
> 
> The Ceph client now supports RWF_DONTCACHE for buffered file IO, but
> it needs preserve the IOCB_DONTCACHE state through
> netfs_write_begin().
> 
> Unlike NFS, Ceph does not need to defer drop-behind until after a
> separate COMMIT step.  Ceph's buffered writeback completes when the
> OSD write completes, so the existing folio_end_writeback() handling is
> sufficient.
> 
> This patch changes netfs because Ceph does not get its write_begin
> folio directly.  ceph_write_begin() is a wrapper around
> netfs_write_begin(), and that helper was calling
> __filemap_get_folio(FGP_WRITEBEGIN, ...) without access to the kiocb.
> As a result, the per-IO IOCB_DONTCACHE flag was lost before folio
> allocation.
> 
> Fix this by passing the kiocb into netfs_write_begin() and switching
> it to write_begin_get_folio(iocb, ...), so IOCB_DONTCACHE is
> translated into FGP_DONTCACHE when appropriate.  Then set
> FOP_DONTCACHE on ceph_file_fops so the VFS will accept RWF_DONTCACHE
> for files on Ceph.
> 
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
>  fs/ceph/addr.c           | 3 ++-
>  fs/ceph/file.c           | 1 +
>  fs/netfs/buffered_read.c | 6 +++---
>  include/linux/netfs.h    | 3 ++-
>  4 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 2090fc78529c..b4ea3ba8211e 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1865,7 +1865,8 @@ static int ceph_write_begin(const struct kiocb *iocb,
>  	struct ceph_inode_info *ci = ceph_inode(inode);
>  	int r;
>  
> -	r = netfs_write_begin(&ci->netfs, file, inode->i_mapping, pos, len, foliop, NULL);
> +	r = netfs_write_begin(iocb, &ci->netfs, file, inode->i_mapping, pos,
> +			      len, foliop, NULL);
>  	if (r < 0)
>  		return r;
>  
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index 5e7c73a29aa3..341589ef32cd 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -3191,4 +3191,5 @@ const struct file_operations ceph_file_fops = {
>  	.compat_ioctl = compat_ptr_ioctl,
>  	.fallocate	= ceph_fallocate,
>  	.copy_file_range = ceph_copy_file_range,
> +	.fop_flags = FOP_DONTCACHE,
>  };
> diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
> index a8c0d86118c5..6ec7c2459d8a 100644
> --- a/fs/netfs/buffered_read.c
> +++ b/fs/netfs/buffered_read.c
> @@ -589,6 +589,7 @@ static bool netfs_skip_folio_read(struct folio *folio, loff_t pos, size_t len,
>  
>  /**
>   * netfs_write_begin - Helper to prepare for writing [DEPRECATED]

The netfs_write_begin() is deprecated function. Maybe, it's time to switch
CephFS on netfs_perform_write() finally?

Thanks,
Slava.

> + * @iocb: The kiocb describing the write request
>   * @ctx: The netfs context
>   * @file: The file to read from
>   * @mapping: The mapping to read from
> @@ -620,7 +621,7 @@ static bool netfs_skip_folio_read(struct folio *folio, loff_t pos, size_t len,
>   * Note that this should be considered deprecated and netfs_perform_write()
>   * used instead.
>   */
> -int netfs_write_begin(struct netfs_inode *ctx,
> +int netfs_write_begin(const struct kiocb *iocb, struct netfs_inode *ctx,
>  		      struct file *file, struct address_space *mapping,
>  		      loff_t pos, unsigned int len, struct folio **_folio,
>  		      void **_fsdata)
> @@ -631,8 +632,7 @@ int netfs_write_begin(struct netfs_inode *ctx,
>  	int ret;
>  
>  retry:
> -	folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
> -				    mapping_gfp_mask(mapping));
> +	folio = write_begin_get_folio(iocb, mapping, index, len);
>  	if (IS_ERR(folio))
>  		return PTR_ERR(folio);
>  
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index ba17ac5bf356..d58c614e32fd 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -414,7 +414,8 @@ int netfs_writeback_single(struct address_space *mapping,
>  struct readahead_control;
>  void netfs_readahead(struct readahead_control *);
>  int netfs_read_folio(struct file *, struct folio *);
> -int netfs_write_begin(struct netfs_inode *, struct file *,
> +int netfs_write_begin(const struct kiocb *iocb,
> +		      struct netfs_inode *ctx, struct file *file,
>  		      struct address_space *, loff_t pos, unsigned int len,
>  		      struct folio **, void **fsdata);
>  int netfs_writepages(struct address_space *mapping,