27056f20d7
commit 73f37dbcfe1763ee2294c7717a1f571e27d17fd8 upstream. When fallocate() is used on a shmem file, the pages we allocate can end up with !PageUptodate. Since UFFDIO_CONTINUE tries to find the existing page the user wants to map with SGP_READ, we would fail to find such a page, since shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it discovers !PageUptodate. As a result, UFFDIO_CONTINUE returns -EFAULT, as it would do if the page wasn't found in the page cache at all. This isn't the intended behavior. UFFDIO_CONTINUE is just trying to find if a page exists, and doesn't care whether it still needs to be cleared or not. So, instead of SGP_READ, pass in SGP_NOALLOC. This is the same, except for one critical difference: in the !PageUptodate case, SGP_NOALLOC will clear the page and then return it. With this change, UFFDIO_CONTINUE works properly (succeeds) on a shmem file which has been fallocated, but otherwise not modified. Link: https://lkml.kernel.org/r/20220610173812.1768919-1-axelrasmussen@google.com Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") Signed-off-by: Axel Rasmussen <axelrasmussen@google.com> Acked-by: Peter Xu <peterx@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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 Restructured Text 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.