On Mon, Nov 29, 2021 at 10:40:38AM -0800, Linus Torvalds 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 have patches for the finer-granularity checking of the beginning of the buffer. They need a bit of testing, so probably posting them tomorrow. > 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 For arm64 at least __put_user() does the access_ok() check. I thought only unsafe_put_user() should skip the checks. If __put_user() can write arbitrary memory, we may have a bigger problem. > (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. Indeed, the fault_in_*() loop can get pretty long, bounded by how much memory can be faulted in the user process. My patches for now only address the outer loop doing the copy_to_user() as that can be unbounded. > 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. I think that would be useful, though it doesn't solve the potential livelock with sub-page faults. We still need the outer loop to handle the copy_to_user() for the whole user buffer and the sub-page probing of the beginning of such buffer (or whenever copy_to_user() failed). IOW, you still fault in the whole buffer eventually. Anyway, I think the sub-page probing and limiting the fault-in are complementary improvements. -- Catalin
