linux-kernel - Re: [PATCH] ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode

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Message-ID: <CAJHvVciZnw5rBApeoAyqW+mLVumUWme+V40eZe7ZNacwuPAZJA@mail.gmail.com>
Date:   Fri, 4 Jun 2021 12:50:17 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>, linux-man@...r.kernel.org,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH] ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode

Thanks for the review, Peter! I'll send a v2 shortly to address the suggestions.

On Thu, Jun 3, 2021 at 12:29 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Thu, Jun 03, 2021 at 11:32:16AM -0700, Axel Rasmussen wrote:
>
> [...]
>
> Not a native speaker, feel free to take anything I said with a grain of salt..
>
> > @@ -278,14 +287,8 @@ by the current kernel version.
> >  (Since Linux 4.3.)
> >  Register a memory address range with the userfaultfd object.
> >  The pages in the range must be "compatible".
> > -.PP
> > -Up to Linux kernel 4.11,
> > -only private anonymous ranges are compatible for registering with
> > -.BR UFFDIO_REGISTER .
> > -.PP
> > -Since Linux 4.11,
> > -hugetlbfs and shared memory ranges are also compatible with
> > -.BR UFFDIO_REGISTER .
> > +What constitutes "compatible" depends on the mode(s) being used, as described
> > +below.
>
> Would below be slightly better?
>
>   Please refer to the list of register modes below for the compatible memory
>   backends for each mode.

I have no significant preference between the two, so happy to reword this one.

>
> [...]
>
> > @@ -735,6 +745,109 @@ or not registered with userfaultfd write-protect mode.
> >  .TP
> >  .B EFAULT
> >  Encountered a generic fault during processing.
> > +.\"
> > +.SS UFFDIO_CONTINUE
> > +(Since Linux 5.13.)
> > +Used for resolving minor faults specifically.
> > +Take the existing page(s) in the range registered with
> > +.B UFFDIO_REGISTER_MODE_MINOR
> > +and install page table entries for them.
>
> "Take the existing page" reads a bit weird to me.  How about something like:
> "Resolving minor-mode trapped page faults by installing page table entries with
> pages in the page cache"?

Agreed, "take" is a bit awkward. I'll reword to something close to
your suggestion.

>
> [...]
>
> > +.TP
> > +.B EINVAL
> > +An invalid bit was specified in the
> > +.IR mode
> > +field.
> > +.TP
> > +.B EEXIST
> > +One or more pages were already mapped in the given range.
>
> I'd think this sentence is good enough; slightly prefer dropping the latter one
> "In other words..." below, as "mapped" should mean the same to me (and the
> wording "fully mapped" is a bit confusing too..).

Fair enough, I had it that way at first but was worried the first
sentence alone was too vague. I'm probably overthinking it. ;) I'll
just drop the second sentence.

>
> > +In other words, not only did pages exist in the page cache, but page table
> > +entries already existed for those pages and they were fully mapped.
>
> [...]
>
> Thanks,
>
> --
> Peter Xu
>