• Andres Lagar-Cavilla's avatar
    kvm: Faults which trigger IO release the mmap_sem · 234b239b
    Andres Lagar-Cavilla authored
    When KVM handles a tdp fault it uses FOLL_NOWAIT. If the guest memory
    has been swapped out or is behind a filemap, this will trigger async
    readahead and return immediately. The rationale is that KVM will kick
    back the guest with an "async page fault" and allow for some other
    guest process to take over.
    
    If async PFs are enabled the fault is retried asap from an async
    workqueue. If not, it's retried immediately in the same code path. In
    either case the retry will not relinquish the mmap semaphore and will
    block on the IO. This is a bad thing, as other mmap semaphore users
    now stall as a function of swap or filemap latency.
    
    This patch ensures both the regular and async PF path re-enter the
    fault allowing for the mmap semaphore to be relinquished in the case
    of IO wait.
    Reviewed-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
    Signed-off-by: default avatarAndres Lagar-Cavilla <andreslc@google.com>
    Acked-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    234b239b
async_pf.c 5.38 KB