Re: [PATCH v3 5/5] shmem, overlayfs, coda, tty, proc, kernfs, random: Fix splice-read

[Miklos Szeredi]

On Tue, 14 Feb 2023 at 09:38, David Howells <dhowells@xxxxxxxxxx> wrote:
>
> The new filemap_splice_read() has an implicit expectation via
> filemap_get_pages() that ->read_folio() exists if ->readahead() doesn't
> fully populate the pagecache of the file it is reading from[1], potentially
> leading to a jump to NULL if this doesn't exist.
>
> A filesystem or driver shouldn't suffer from this if:
>
>   - It doesn't set ->splice_read()
>   - It implements ->read_folio()
>   - It implements its own ->splice_read()
>
> Note that some filesystems set generic_file_splice_read() and
> generic_file_read_iter() but don't set ->read_folio().  g_f_read_iter()
> will fall back to filemap_read_iter() which looks like it should suffer
> from the same issue.
>
> Certain drivers, can just use direct_splice_read() rather than
> generic_file_splice_read() as that creates an output buffer and then just
> calls their ->read_iter() function:
>
>   - random & urandom
>   - tty
>   - kernfs
>   - proc
>   - proc_namespace
>
> Stacked filesystems just need to pass the operation down a layer:
>
>   - coda
>   - overlayfs
>
> And finally, there's shmem (used in tmpfs, ramfs, rootfs).  This needs its
> own splice-read implementation, based on filemap_splice_read(), but able to
> paste in zero_page when there's a page missing.
>
> Fixes: d9722a475711 ("splice: Do splice read from a buffered file without using ITER_PIPE")

The fixed commit is not upstream.  In fact it seems to be on the same
branch as this one. Please reorder the patches so that a Fixes tag is
not needed.

Thanks,
Miklos