From: Andrea Arcangeli <aarcange@xxxxxxxxxx> 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@xxxxxxxxxx Fixes: ce53053ce378c ("kvm: switch get_user_page_nowait() to get_user_pages_unlocked()") Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> Reported-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> Tested-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- 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; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
