• Linus Torvalds's avatar
    Manually revert "mlock: downgrade mmap sem while populating mlocked regions" · 27421e21
    Linus Torvalds authored
    This essentially reverts commit 8edb08ca.
    
    It downgraded our mmap semaphore to a read-lock while mlocking pages, in
    order to allow other threads (and external accesses like "ps" et al) to
    walk the vma lists and take page faults etc.  Which is a nice idea, but
    the implementation does not work.
    
    Because we cannot upgrade the lock back to a write lock without
    releasing the mmap semaphore, the code had to release the lock entirely
    and then re-take it as a writelock.  However, that meant that the caller
    possibly lost the vma chain that it was following, since now another
    thread could come in and mmap/munmap the range.
    
    The code tried to work around that by just looking up the vma again and
    erroring out if that happened, but quite frankly, that was just a buggy
    hack that doesn't actually protect against anything (the other thread
    could just have replaced the vma with another one instead of totally
    unmapping it).
    
    The only way to downgrade to a read map _reliably_ is to do it at the
    end, which is likely the right thing to do: do all the 'vma' operations
    with the write-lock held, then downgrade to a read after completing them
    all, and then do the "populate the newly mlocked regions" while holding
    just the read lock.  And then just drop the read-lock and return to user
    space.
    
    The (perhaps somewhat simpler) alternative is to just make all the
    callers of mlock_vma_pages_range() know that the mmap lock got dropped,
    and just re-grab the mmap semaphore if it needs to mlock more than one
    vma region.
    
    So we can do this "downgrade mmap sem while populating mlocked regions"
    thing right, but the way it was done here was absolutely not correct.
    Thus the revert, in the expectation that we will do it all correctly
    some day.
    
    Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: stable@kernel.org
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    27421e21
mlock.c 16.5 KB