On Tuesday, April 27th, 2021 at 6:51 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > Hmm. It doesn't look too hard to do. The biggest problem is actually > that we've run out of flags in the vma (on 32-bit architectures), but > you could try this UNTESTED patch that just does the MAP_NOFAULT thing > unconditionally. Oh, thanks for the patch! Will test. > Side note: this will only ever work for non-shared mappings. That's > fundamental. We won't add an anonymous page to a shared mapping, and > do_anonymous_page() does verify that. So a MAP_SHARED mappign will > still return SIGBUS even with this patch (although it's not obvious > from the patch - the VM_FAULT_SIGBUS will just be re-created by > do_anonymous_page()). > > So if you want a _shared_ mapping to honor __MAP_NOFAULT and insert > random anonymous pages into it, I think the answer is "no, that's not > going to be viable". > > So _if_ this works for you, and if it's ok that only MAP_PRIVATE can > have __MAP_NOFAULT, and if Kirill/Peter/Will don't say "Oh, Linus, > you're completely off your rocker and clearly need to be taking your > meds", something like this - if we figure out the conditional bit - > might be doable. Hm, that's unfortunate. For the use-case of a Wayland compositor this doesn't seem like a complete show-stopper: in 90% of cases the compositor only needs a read-only mapping. Wayland clients submit buffers they're rendered pixels to, compositors only need to read them. So the compositor could map with MAP_PRIVATE and still get up-to-date pages from a client process I think. The remaining 10% is when the compositor needs a writable mapping for things like screen capture. It doesn't seem like a SIGBUS handler can be avoided in this case then… Oh well. > That's a fair number of "ifs". > > Ok, back to the merge window for me, I'll be throwing away this crazy > untested patch immediately after hitting "send". This is very much a > "throw the idea over to other people" patch, in other words. Got it. I'll take over the patch if this is a good way forward.
