On Thu, Jun 03, 2021 at 01:14:47PM +0000, Simon Ser wrote: > On Saturday, May 29th, 2021 at 10:15 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > And IIUC it would have to be the recipient (Wayland compositor) doing > > the NOFAULT business, because (going back to the original mail) we are > > only considering this so that Wayland might satisfy clients who predate > > or refuse Linux-only APIs. So, an ioctl (or fcntl, as sealing chose) > > at the client end cannot be expected; and could not be relied on anyway. > > Yes, that is correct. > > > NOFAULT? Does BSD use "fault" differently, and in Linux terms we > > would say NOSIGBUS to mean the same? > > > > Can someone point to a specification of BSD's __MAP_NOFAULT? > > Searching just found me references to bugs. > > __MAP_NOFAULT isn't documented, sadly. The commit that introduces the > flag [1] is the best we're going to get, I think. > > > What mainly worries me about the suggestion is: what happens to the > > zero page inserted into NOFAULT mappings, when later a page for that > > offset is created and added to page cache? > > Not 100% sure exactly this means what I think it means, but from my PoV, > it's fine if the contents of an expanded shm file aren't visible from the > process that has mapped it with MAP_NOFAULT/MAP_NOSIGBUS. In other words, > it's fine if: > > - The client sets up a 1KiB shm file and sends it to the compositor. > - The compositor maps it with MAP_NOFAULT/MAP_NOSIGBUS. > - The client expands the file to 2KiB and writes interesting data in it. > - The compositor still sees zeros past the 1KiB mark. The compositor needs > to unmap and re-map the file to see the data past the 1KiB mark. > > If the MAP_NOFAULT/MAP_NOSIGBUS flag only affects the mapping itself and > nothing else, this should be fine? This is going to operate at a page boundary, so the example you gave will work. How about this: - The client sets up a 1KiB shm file and sends it to the compositor. - The client expands the file to 5KiB - The compositor sees the new data up to 4KiB but zeroes past the 4KiB mark. Does that still make userspace happy?
