The patch titled Subject: mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT has been added to the -mm tree. Its filename is mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait.patch
This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-gup-teach-get_user_pages_unlocke... and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-gup-teach-get_user_pages_unlocke...
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------------------------------------------------------ From: Andrea Arcangeli aarcange@redhat.com Subject: mm/gup.c: teach get_user_pages_unlocked to handle FOLL_NOWAIT
KVM is hanging during postcopy live migration with userfaultfd because get_user_pages_unlocked is not capable to handle FOLL_NOWAIT.
Earlier FOLL_NOWAIT was only ever passed to get_user_pages.
Specifically faultin_page (the callee of get_user_pages_unlocked caller) doesn't know that if FAULT_FLAG_RETRY_NOWAIT was set in the page fault flags, when VM_FAULT_RETRY is returned, the mmap_sem wasn't actually released (even if nonblocking is not NULL). So it sets *nonblocking to zero and the caller won't release the mmap_sem thinking it was already released, but it wasn't because of FOLL_NOWAIT.
Link: http://lkml.kernel.org/r/20180302174343.5421-2-aarcange@redhat.com Fixes: ce53053ce378c ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()") Signed-off-by: Andrea Arcangeli aarcange@redhat.com Reported-by: Dr. David Alan Gilbert dgilbert@redhat.com Tested-by: Dr. David Alan Gilbert dgilbert@redhat.com Cc: Al Viro viro@zeniv.linux.org.uk Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/gup.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)
diff -puN mm/gup.c~mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait mm/gup.c --- a/mm/gup.c~mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait +++ a/mm/gup.c @@ -516,7 +516,7 @@ static int faultin_page(struct task_stru }
if (ret & VM_FAULT_RETRY) { - if (nonblocking) + if (nonblocking && !(fault_flags & FAULT_FLAG_RETRY_NOWAIT)) *nonblocking = 0; return -EBUSY; } @@ -890,7 +890,10 @@ static __always_inline long __get_user_p break; } if (*locked) { - /* VM_FAULT_RETRY didn't trigger */ + /* + * VM_FAULT_RETRY didn't trigger or it was a + * FOLL_NOWAIT. + */ if (!pages_done) pages_done = ret; break; _
Patches currently in -mm which might be from aarcange@redhat.com are
mm-gup-teach-get_user_pages_unlocked-to-handle-foll_nowait.patch