• Peter Xu's avatar
    mm: allow VM_FAULT_RETRY for multiple times · 4064b982
    Peter Xu authored
    The idea comes from a discussion between Linus and Andrea [1].
    
    Before this patch we only allow a page fault to retry once.  We achieved
    this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing
    handle_mm_fault() the second time.  This was majorly used to avoid
    unexpected starvation of the system by looping over forever to handle the
    page fault on a single page.  However that should hardly happen, and after
    all for each code path to return a VM_FAULT_RETRY we'll first wait for a
    condition (during which time we should possibly yield the cpu) to happen
    before VM_FAULT_RETRY is really returned.
    
    This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY
    flag when we receive VM_FAULT_RETRY.  It means that the page fault handler
    now can retry the page fault for multiple times if necessary without the
    need to generate another page fault event.  Meanwhile we still keep the
    FAULT_FLAG_TRIED flag so page fault handler can still identify whether a
    page fault is the first attempt or not.
    
    Then we'll have these combinations of fault flags (only considering
    ALLOW_RETRY flag and TRIED flag):
    
      - ALLOW_RETRY and !TRIED:  this means the page fault allows to
                                 retry, and this is the first try
    
      - ALLOW_RETRY and TRIED:   this means the page fault allows to
                                 retry, and this is not the first try
    
      - !ALLOW_RETRY and !TRIED: this means the page fault does not allow
                                 to retry at all
    
      - !ALLOW_RETRY and TRIED:  this is forbidden and should never be used
    
    In existing code we have multiple places that has taken special care of
    the first condition above by checking against (fault_flags &
    FAULT_FLAG_ALLOW_RETRY).  This patch introduces a simple helper to detect
    the first retry of a page fault by checking against both (fault_flags &
    FAULT_FLAG_ALLOW_RETRY) and !(fault_flag & FAULT_FLAG_TRIED) because now
    even the 2nd try will have the ALLOW_RETRY set, then use that helper in
    all existing special paths.  One example is in __lock_page_or_retry(), now
    we'll drop the mmap_sem only in the first attempt of page fault and we'll
    keep it in follow up retries, so old locking behavior will be retained.
    
    This will be a nice enhancement for current code [2] at the same time a
    supporting material for the future userfaultfd-writeprotect work, since in
    that work there will always be an explicit userfault writeprotect retry
    for protected pages, and if that cannot resolve the page fault (e.g., when
    userfaultfd-writeprotect is used in conjunction with swapped pages) then
    we'll possibly need a 3rd retry of the page fault.  It might also benefit
    other potential users who will have similar requirement like userfault
    write-protection.
    
    GUP code is not touched yet and will be covered in follow up patch.
    
    Please read the thread below for more information.
    
    [1] https://lore.kernel.org/lkml/20171102193644.GB22686@redhat.com/
    [2] https://lore.kernel.org/lkml/20181230154648.GB9832@redhat.com/Suggested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Suggested-by: default avatarAndrea Arcangeli <aarcange@redhat.com>
    Signed-off-by: default avatarPeter Xu <peterx@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Tested-by: default avatarBrian Geffon <bgeffon@google.com>
    Cc: Bobby Powers <bobbypowers@gmail.com>
    Cc: David Hildenbrand <david@redhat.com>
    Cc: Denis Plotnikov <dplotnikov@virtuozzo.com>
    Cc: "Dr . David Alan Gilbert" <dgilbert@redhat.com>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Jerome Glisse <jglisse@redhat.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: "Kirill A . Shutemov" <kirill@shutemov.name>
    Cc: Martin Cracauer <cracauer@cons.org>
    Cc: Marty McFadden <mcfadden8@llnl.gov>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Maya Gokhale <gokhale2@llnl.gov>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Mike Kravetz <mike.kravetz@oracle.com>
    Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
    Cc: Pavel Emelyanov <xemul@openvz.org>
    Link: http://lkml.kernel.org/r/20200220160246.9790-1-peterx@redhat.comSigned-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    4064b982
fault.c 6.64 KB