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 Cc: Rik van Riel Cc: Andrew Morton Cc: stable@kernel.org Signed-off-by: Linus Torvalds