On 19.03.22 00:30, Jason Gunthorpe wrote: > On Tue, Mar 15, 2022 at 11:47:39AM +0100, David Hildenbrand wrote: >> Whenever GUP currently ends up taking a R/O pin on an anonymous page that >> might be shared -- mapped R/O and !PageAnonExclusive() -- any write fault >> on the page table entry will end up replacing the mapped anonymous page >> due to COW, resulting in the GUP pin no longer being consistent with the >> page actually mapped into the page table. >> >> The possible ways to deal with this situation are: >> (1) Ignore and pin -- what we do right now. >> (2) Fail to pin -- which would be rather surprising to callers and >> could break user space. >> (3) Trigger unsharing and pin the now exclusive page -- reliable R/O >> pins. >> >> We want to implement 3) because it provides the clearest semantics and >> allows for checking in unpin_user_pages() and friends for possible BUGs: >> when trying to unpin a page that's no longer exclusive, clearly >> something went very wrong and might result in memory corruptions that >> might be hard to debug. So we better have a nice way to spot such >> issues. >> >> To implement 3), we need a way for GUP to trigger unsharing: >> FAULT_FLAG_UNSHARE. FAULT_FLAG_UNSHARE is only applicable to R/O mapped >> anonymous pages and resembles COW logic during a write fault. However, in >> contrast to a write fault, GUP-triggered unsharing will, for example, still >> maintain the write protection. > > Given the way this series has developed you might want to call this > FAULT_FLAG_MAKE_ANON_EXCLUSIVE > > Which strikes me as more directly connected to what it is trying to > do. I thought about something similar along those lines, and I think it would apply even when extending that mechanism to anything !anon inside a MAP_PRIVATE mapping. The whole const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE; would get a little more verbose, though const bool make_anon_exclusive = vmf->flags & FAULT_FLAG_MAKE_ANON_EXCLUSIVE; No strong opinion, any other opinions or alternatives? -- Thanks, David / dhildenb
