Re: [PATCH v5 33/40] netfs, cachefiles: Pass upper bound length to allow expansion

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi David,

On 2023/12/21 21:23, David Howells wrote:
> Make netfslib pass the maximum length to the ->prepare_write() op to tell
> the cache how much it can expand the length of a write to.  This allows a
> write to the server at the end of a file to be limited to a few bytes
> whilst writing an entire block to the cache (something required by direct
> I/O).
> 
> Signed-off-by: David Howells <dhowells@redhat.com>
> Reviewed-by: Jeff Layton <jlayton@kernel.org>
> cc: linux-cachefs@redhat.com
> cc: linux-fsdevel@vger.kernel.org
> cc: linux-mm@kvack.org
> ---
>   fs/cachefiles/internal.h |  2 +-
>   fs/cachefiles/io.c       | 10 ++++++----
>   fs/cachefiles/ondemand.c |  2 +-
>   fs/netfs/fscache_io.c    |  2 +-
>   fs/netfs/io.c            |  2 +-
>   fs/netfs/objects.c       |  1 +
>   fs/netfs/output.c        | 25 ++++++++++---------------
>   fs/smb/client/fscache.c  |  2 +-
>   include/linux/netfs.h    |  5 +++--
>   9 files changed, 25 insertions(+), 26 deletions(-)
> 
> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
> index 2ad58c465208..1af48d576a34 100644
> --- a/fs/cachefiles/internal.h
> +++ b/fs/cachefiles/internal.h
> @@ -233,7 +233,7 @@ extern bool cachefiles_begin_operation(struct netfs_cache_resources *cres,
>   				       enum fscache_want_state want_state);
>   extern int __cachefiles_prepare_write(struct cachefiles_object *object,
>   				      struct file *file,
> -				      loff_t *_start, size_t *_len,
> +				      loff_t *_start, size_t *_len, size_t upper_len,
>   				      bool no_space_allocated_yet);
>   extern int __cachefiles_write(struct cachefiles_object *object,
>   			      struct file *file,
> diff --git a/fs/cachefiles/io.c b/fs/cachefiles/io.c
> index 009d23cd435b..bffffedce4a9 100644
> --- a/fs/cachefiles/io.c
> +++ b/fs/cachefiles/io.c
> @@ -518,7 +518,7 @@ cachefiles_prepare_ondemand_read(struct netfs_cache_resources *cres,
>    */
>   int __cachefiles_prepare_write(struct cachefiles_object *object,
>   			       struct file *file,
> -			       loff_t *_start, size_t *_len,
> +			       loff_t *_start, size_t *_len, size_t upper_len,
>   			       bool no_space_allocated_yet)
>   {
>   	struct cachefiles_cache *cache = object->volume->cache;
> @@ -530,6 +530,8 @@ int __cachefiles_prepare_write(struct cachefiles_object *object,
>   	down = start - round_down(start, PAGE_SIZE);
>   	*_start = start - down;
>   	*_len = round_up(down + len, PAGE_SIZE);
> +	if (down < start || *_len > upper_len)
> +		return -ENOBUFS;

Sorry for bothering. We just found some strange when testing
today-next EROFS over fscache.

I'm not sure the meaning of
     if (down < start

For example, if start is page-aligned, down == 0.

so as long as start > 0 and page-aligned, it will return
-ENOBUFS.  Does it an intended behavior?

Thanks,
Gao Xiang