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

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Date:   Tue, 14 Feb 2023 14:05:49 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     David Howells <dhowells@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Daniel Golle <daniel@...rotopia.org>,
        Guenter Roeck <groeck7@...il.com>,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jan Harkes <jaharkes@...cmu.edu>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        coda@...cmu.edu, codalist@...a.cs.cmu.edu,
        linux-unionfs@...r.kernel.org
Subject: Re: [PATCH v3 5/5] shmem, overlayfs, coda, tty, proc, kernfs, random:
 Fix splice-read

On Tue, 14 Feb 2023 at 09:38, David Howells <dhowells@...hat.com> 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