mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount
authorZach O'Keefe <zokeefe@google.com>
Wed, 25 Jan 2023 01:57:37 +0000 (17:57 -0800)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Wed, 22 Feb 2023 11:59:49 +0000 (12:59 +0100)
commitfd71c8d3b005f0f5047f0c7480ee3bba12b76aa9
tree6af561fe7f27e54f04d22b2b4f852734be9e58cf
parenta8ef5109f93cea9933bbac0455d8c18757b3fcb4
mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

commit ae63c898f4004bbc7d212f4adcb3bb14852c30d6 upstream.

During collapse, in a few places we check to see if a given small page has
any unaccounted references.  If the refcount on the page doesn't match our
expectations, it must be there is an unknown user concurrently interested
in the page, and so it's not safe to move the contents elsewhere.
However, the unaccounted pins are likely an ephemeral state.

In this situation, MADV_COLLAPSE returns -EINVAL when it should return
-EAGAIN.  This could cause userspace to conclude that the syscall
failed, when it in fact could succeed by retrying.

Link: https://lkml.kernel.org/r/20230125015738.912924-1-zokeefe@google.com
Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
Signed-off-by: Zach O'Keefe <zokeefe@google.com>
Reported-by: Hugh Dickins <hughd@google.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
mm/khugepaged.c