Messages in this thread | | | Date | Wed, 29 Jun 2022 18:53:30 -0700 | Subject | Re: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE | From | John Hubbard <> |
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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.
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. */
?
>> 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. :)
>> >> 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.
thanks, -- John Hubbard NVIDIA
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