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When a read subrequest finishes, if it doesn't have sufficient coverage to complete the folio(s) covering either side of it, it will donate the excess coverage to the adjacent subrequests on either side, offloading responsibility for unlocking the folio(s) covered to them. Now, preference is given to donating down to a lower file offset over donating up because that check is done first - but there's no check that the lower subreq is actually contiguous, and so we can end up donating incorrectly. The scenario seen[1] is that an 8MiB readahead request spanning four 2MiB folios is split into eight 1MiB subreqs (numbered 1 through 8). These terminate in the order 1,6,2,5,3,7,4,8. What happens is: - 1 donates to 2 - 6 donates to 5 - 2 completes, unlocking the first folio (with 1). - 5 completes, unlocking the third folio (with 6). - 3 donates to 4 - 7 donates to 4 incorrectly - 4 completes, unlocking the second folio (with 3), but can't use the excess from 7. - 8 donates to 4, also incorrectly. Fix this by preventing downward donation if the subreqs are not contiguous (in the example above, 7 donates to 4 across the gap left by 5 and 6). Reported-by: Shyam Prasad N <nspmangalore@gmail.com> Closes: https://lore.kernel.org/r/CANT5p=qBwjBm-D8soFVVtswGEfmMtQXVW83=TNfUtvyHeFQZBA@mail.gmail.com/ Signed-off-by: David Howells <dhowells@redhat.com> Link: https://lore.kernel.org/r/526707.1733224486@warthog.procyon.org.uk/ [1] Link: https://lore.kernel.org/r/20241213135013.2964079-3-dhowells@redhat.com cc: Steve French <sfrench@samba.org> cc: Paulo Alcantara <pc@manguebit.com> cc: Jeff Layton <jlayton@kernel.org> cc: linux-cifs@vger.kernel.org cc: netfs@lists.linux.dev cc: linux-fsdevel@vger.kernel.org Signed-off-by: Christian Brauner <brauner@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.