On Mon, Nov 29, 2021 at 7:41 PM Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Mon, Nov 29, 2021 at 7:36 AM Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > > > That's what this series does when it probes the whole range in > > fault_in_writeable(). The main reason was that it's more efficient to do > > a read than a write on a large range (the latter dirtying the cache > > lines). > > The more this thread goes on, the more I'm starting to think that we > should just make "fault_in_writable()" (and readable, of course) only > really work on the beginning of the area. > > Not just for the finer-granularity pointer color probing, but for the > page probing too. > > I'm looking at our current fault_in_writeable(), and I'm going > > (a) it uses __put_user() without range checks, which is really not great > > (b) it looks like a disaster from another standpoint: essentially > user-controlled loop size with no limit checking, no preemption, and > no check for fatal signals. > > Now, (a) should be fixed with a access_ok() or similar. > > And (b) can easily be fixed multiple ways, with one option simply just > being adding a can_resched() call and checking for fatal signals. > > But faulting in the whole region is actually fundamentally wrong in > low-memory situations - the beginning of the region might be swapped > out by the time we get to the end. That's unlikely to be a problem in > real life, but it's an example of how it's simply not conceptually > sensible. > > So I do wonder why we don't just say "fault_in_writable will fault in > _at_most_ X bytes", and simply limit the actual fault-in size to > something reasonable. > > That solves _all_ the problems. It solves the lack of preemption and > fatal signals (by virtue of just limiting the amount of work we do). > It solves the low memory situation. And it solves the "excessive dirty > cachelines" case too. > > Of course, we want to have some minimum bytes we fault in too, but > that minimum range might well be "we guarantee at least a full page > worth of data" (and in practice make it a couple of pages). > > It's not like fault_in_writeable() avoids page faults or anything like > that - it just moves them around. So there's really very little reason > to fault in a large range, and there are multiple reasons _not_ to do > it. > > Hmm? This would mean that we could get rid of gfs2's should_fault_in_pages() logic, which is based on what's in btrfs_buffered_write(). Andreas > > Linus >