Re: [PATCH v2] fuse: Forward vfs_fadvise to backing_file in passthrough mode

[T.J. Mercier]

On Thu, Sep 10, 2026 at 11:45 AM Shai Barack <shayba@xxxxxxxxxx> wrote:
>
> When an application invokes posix_fadvise(..., POSIX_FADV_DONTNEED) or
> other fadvise advice on a FUSE passthrough file descriptor, the VFS
> currently only executes generic_fadvise() on the upper FUSE inode
> mapping.
>
> Because FUSE passthrough does not forward fadvise operations to the
> underlying backing file, pages residing in the lower filesystem's
> pagecache remain resident in memory.
>
> During media-heavy workloads (such as image thumbnail generation or
> streaming), single-access file data accumulates in the inactive pagecache
> of the lower filesystem. Under memory pressure, the kernel retains these
> stale media pages and instead evicts active executable pages from
> running processes, leading to severe direct reclaim latency spikes.
>
> Architecturally, Linux VFS supports filesystem-specific fadvise
> delegation via the .fadvise hook in struct file_operations. Implement
> fuse_file_fadvise() in fs/fuse/file.c and register it in
> fuse_file_operations. When passthrough is active on a fuse_file,
> fuse_file_fadvise() forwards the vfs_fadvise() call to the lower
> backing_file under the opened credentials, allowing POSIX_FADV_DONTNEED
> to cleanly invalidate the physical backing filesystem pagecache.
>
> Signed-off-by: Shai Barack <shayba@xxxxxxxxxx>

Reviewed-by: T.J. Mercier <tjmercier@xxxxxxxxxx>