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

Gao Xiang
2024-01-02T14:04:23Z

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