Re: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

[Peter Xu <peterx@redhat.com>]

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