On Wed, Jun 29, 2022 at 06:53:30PM -0700, John Hubbard wrote: > On 6/29/22 08:47, Peter Xu wrote: > > > It looks like part of this comment is trying to document a pre-existing > > > concept, which is that faultin_page() only ever sets FAULT_FLAG_KILLABLE > > > if locked != NULL. > > > > I'd say that's not what I wanted to comment.. I wanted to express that > > INTERRUPTIBLE should rely on KILLABLE, that's also why I put the comment to > > be after KILLABLE, not before. IMHO it makes sense already to have > > "interruptible" only if "killable", no matter what's the pre-requisite for > > KILLABLE (in this case it's having "locked" being non-null). > > > > OK, I think I finally understand both the intention of the comment, > and (thanks to your notes, below) the interaction between *locked and > _RETRY, _KILLABLE, and _INTERRUPTIBLE. Really appreciate your leading > me by the nose through that. The pre-existing code is abusing *locked > a bit, by treating it as a flag when really it is a side effect of > flags, but at least now that's clear to me. I agree, alternatively we could have some other FOLL_ flags to represent "locked != NULL" and do sanity check to make sure when the flag is there locked is always set correctly. Current code is a more "dense" way to do this, even though it could be slightly harder to follow. > > Anyway...this leads to finally getting into the comment, which I now > think is not quite what we want: there is no need for a hierarchy of > "_INTERRUPTIBLE should depend upon _KILLABLE". That is: even though an > application allows a fatal signal to get through, it's not clear to me > that that implies that non-fatal signal handling should be prevented. > > The code is only vaguely enforcing such a thing, because it just so > happens that both cases require the same basic prerequisites. So the > code looks good, but I don't see a need to claim a hierarchy in the > comments. > > So I'd either delete the comment entirely, or go with something that is > doesn't try to talk about hierarchy nor locked/retry either. Does this > look reasonable to you: > > > /* > * FAULT_FLAG_INTERRUPTIBLE is opt-in: kernel callers must set > * FOLL_INTERRUPTIBLE. That's because some callers may not be > * prepared to handle early exits caused by non-fatal signals. > */ > > ? Looks good to me, I'd tune a bit to make it less ambiguous on a few places: /* * FAULT_FLAG_INTERRUPTIBLE is opt-in. GUP callers must set * FOLL_INTERRUPTIBLE to enable FAULT_FLAG_INTERRUPTIBLE. * That's because some callers may not be prepared to * handle early exits caused by non-fatal signals. */ Would that be okay to you? > > > > The problem I am (personally) having is that I don't yet understand why > > > or how those are connected: what is it about having locked non-NULL that > > > means the process is killable? (Can you explain why that is?) > > > > Firstly RETRY_KILLABLE relies on ALLOW_RETRY, because if we don't allow > > retry at all it means we'll never wait in handle_mm_fault() anyway, then no > > need to worry on being interrupted by any kind of signal (fatal or not). > > > > Then if we allow retry, we need some way to know "whether mmap_sem is > > released or not" during the process for the caller (because the caller > > cannot see VM_FAULT_RETRY). That's why we added "locked" parameter, so > > that we can set *locked=false to tell the caller we have released mmap_sem. > > > > I think that's why we have "locked" defined as "we allow this page fault > > request to retry and wait, during wait we can always allow fatal signals". > > I think that's defined throughout the gup call interfaces too, and > > faultin_page() is the last step to talk to handle_mm_fault(). > > > > To make this whole picture complete, NOWAIT is another thing that relies on > > ALLOW_RETRY but just to tell "oh please never release the mmap_sem at all". > > For example, when we want to make sure no vma will be released after > > faultin_page() returned. > > > > Again, thanks for taking the time to explain that for me. :) My thanks for reviewing! > > > > > > > If that were clear, I think I could suggest a good comment wording. > > > > IMHO it's a little bit weird to explain "locked" here, especially after > > KILLABLE is set, that's why I didn't try to mention "locked" in my 2nd > > attempt. There are some comments for "locked" above the definition of > > faultin_page(), I think that'll be a nicer place to enrich explanations for > > "locked", and it seems even more suitable as a separate patch? > > > > Totally agreed. I didn't intend to ask for that kind of documentation > here. > > For that, I'm thinking a combination of cleaning up *locked a little > bit, plus maybe some higher level notes like what you wrote above, added > to either pin_user_pages.rst or a new get_user_pages.rst or some .rst > anyway. Definitely a separately thing. Sounds good. Thanks, -- Peter Xu
