On Tue, Jun 22, 2021 at 03:27:43PM +0000, Al Viro wrote: > On Tue, Jun 22, 2021 at 04:20:40PM +0100, David Howells wrote: > > > and wondering if the iov_iter_fault_in_readable() is actually effective. Yes, > > it can make sure that the page we're intending to modify is dragged into the > > pagecache and marked uptodate so that it can be read from, but is it possible > > for the page to then get reclaimed before we get to > > iov_iter_copy_from_user_atomic()? a_ops->write_begin() could potentially take > > a long time, say if it has to go and get a lock/lease from a server. > > Yes, it is. So what? We'll just retry. You *can't* take faults while holding > some pages locked; not without shitloads of deadlocks. Note that the revert you propose is going to do fault-in anyway; we really can't avoid it. The only thing it does is optimistically trying without that the first time around, which is going to be an overall loss exactly in "slow write_begin" case. If source pages are absent, you'll get copyin fail; iov_iter_copy_from_user_atomic() (or its replacement) is disabling pagefaults itself.
