Re: [Qemu-devel] [PATCH 1/1] mm: gup: teach get_user_pages_unlocked to handle FOLL_NOWAIT

[Andrew Morton]

On Fri,  2 Mar 2018 18:43:43 +0100 Andrea Arcangeli <address@hidden> wrote:

> 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.
> 
> Reported-by: Dr. David Alan Gilbert <address@hidden>
> Tested-by: Dr. David Alan Gilbert <address@hidden>
> Signed-off-by: Andrea Arcangeli <address@hidden>

I added

Fixes: ce53053ce378c ("kvm: switch get_user_page_nowait() to 
get_user_pages_unlocked()")
Cc: <address@hidden>