linux-kernel - Re: [PATCH 1/4] mm/gup: Add FOLL

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Date:   Wed, 29 Jun 2022 11:47:29 -0400
From:   Peter Xu <peterx@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 1/4] mm/gup: Add FOLL_INTERRUPTIBLE

On Tue, Jun 28, 2022 at 05:31:43PM -0700, John Hubbard wrote:
> On 6/28/22 15:33, Peter Xu wrote:
> > > The key point is the connection between "locked" and killable. If the comment
> > > explained why "locked" means "killable", that would help clear this up. The
> > > NOWAIT sentence is also confusing to me, and adding "mostly NOWAIT" does not
> > > clear it up either... :)
> > 
> > Sorry to have a comment that makes it feels confusing.  I tried to
> > explicitly put the comment to be after setting FAULT_FLAG_KILLABLE but
> > obviously I didn't do my job well..
> > 
> > Maybe that NOWAIT thing adds more complexity but not even necessary.
> > 
> > Would below one more acceptable?
> > 
> > 		/*
> > 		 * We'll only be able to respond to signals when "locked !=
> > 		 * NULL".  When with it, we'll always respond to SIGKILL
> > 		 * (as implied by FAULT_FLAG_KILLABLE above), and we'll
> > 		 * respond to non-fatal signals only if the GUP user has
> > 		 * specified FOLL_INTERRUPTIBLE.
> > 		 */
> 
> 
> 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).

> 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.

> 
> 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?

Thanks,

-- 
Peter Xu