On 22.12.21 09:51, David Hildenbrand wrote: > On 21.12.21 20:07, Jason Gunthorpe wrote: >> On Tue, Dec 21, 2021 at 06:40:30PM +0100, David Hildenbrand wrote: >> >>> 2) is certainly the cherry on top. But it just means that R/O pins don't >>> have to be the weird kid. And yes, achieving 2) would require >>> FAULT_FLAG_EXCLUSIVE / FAULT_FLAG_UNSHARED, but it would really 99% do >>> what existing COW logic does, just bypass the "map writable" and >>> "trigger write fault" semantics. >> >> I still don't agree with this - when you come to patches can you have >> this work at the end and under a good cover letter? Maybe it will make >> more sense then. > > Yes. But really, I think it's the logical consequence of what Linus said > [1]: > > "And then all GUP-fast would need to do is to refuse to look up a page > that isn't exclusive to that VM. We already have the situation that > GUP-fast can fail for non-writable pages etc, so it's just another > test." > > We must not FOLL_PIN a page that is not exclusive (not only on gup-fast, > but really, on any gup). If we special case R/O FOLL_PIN, we cannot > enable the sanity check on unpin as suggested by Linus [2]: > > "If we only set the exclusive VM bit on pages that get mapped into > user space, and we guarantee that GUP only looks up such pages, then > we can also add a debug test to the "unpin" case that the bit is > still set." > > There are really only two feasible options I see when we want to take a > R/O FOLL_PIN on a !PageAnonExclusive() anon page > > (1) Fail the pinning completely. This implies that we'll have to fail > O_DIRECT once converted to FOLL_PIN. > (2) Request to mark the page PageAnonExclusive() via a > FAULT_FLAG_UNSHARE and let it succeed. > > > Anything else would require additional accounting that we already > discussed in the past is hard -- for example, to differentiate R/O from > R/W pins requiring two pin counters. > > The only impact would be that FOLL_PIN after fork() has to go via a > FAULT_FLAG_UNSHARE once, to turn the page PageAnonExclusive. IMHO this > is the right thing to do for FOLL_LONGTERM. For !FOLL_LONGTERM it would > be nice to optimize this, to *not* do that, but again ... this would > require even more counters I think, for example, to differentiate > between "R/W short/long-term or R/O long-term pin" and "R/O short-term pin". > > So unless we discover a way to do additional accounting for ordinary 4k > pages, I think we really can only do (1) or (2) to make sure we never > ever pin a !PageAnonExclusive() page. BTW, I just wondered if the optimization should actually be that R/O short-term FOLL_PIN users should actually be using FOLL_GET instead. So O_DIRECT with R/O would already be doing the right thing. And it somewhat aligns with what we found: only R/W short-term FOLL_GET is problematic, where we can lose writes to the page from the device via O_DIRECT. IIUC, our COW logic makes sure that a shared anonymous page that might still be used by a R/O FOLL_GET cannot be modified, because any attempt to modify it would result in a copy. But I might be missing something, just an idea. -- Thanks, David / dhildenb
