From: Linus Torvalds <torvalds@linux-foundation.org>
To: Peter Xu <peterx@redhat.com>
Cc: Linux-MM <linux-mm@kvack.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Marty Mcfadden <mcfadden8@llnl.gov>,
Andrea Arcangeli <aarcange@redhat.com>,
Jann Horn <jannh@google.com>, Christoph Hellwig <hch@lst.de>,
Oleg Nesterov <oleg@redhat.com>,
Kirill Shutemov <kirill@shutemov.name>, Jan Kara <jack@suse.cz>
Subject: Re: [PATCH v2] mm/gup: Allow real explicit breaking of COW
Date: Mon, 10 Aug 2020 16:19:07 -0700 [thread overview]
Message-ID: <CAHk-=whng9zK-GMM+UHsTptF=mgH164Zt_Gok3ohbcWaYee71Q@mail.gmail.com> (raw)
In-Reply-To: <20200810215734.GB132381@xz-x1>
On Mon, Aug 10, 2020 at 2:57 PM Peter Xu <peterx@redhat.com> wrote:
>
> Yeah, that's why I totally agree we need to do enforced COW even for a read gup
> as long as the page can be further referenced (GET|PIN). However frankly
> speaking I didn't follow the rest on what's wrong with "Userfaultfd-wp should
> not care about this because it's not a write operation" that I mentiond. Is
> that the major part of the objection?
You didn't _explain_ why it's ok.
You said "it's only reading".
And I told you why "only reading" is not an argument against COW.
So my objection is that you haven't actually explained why it's ok for
userfaultfd-wp.
I can easily believe that it is, but my whole point is that this is
subtle, and it needs documentation and explanation.
That's also why I think the bit should be the other way: the people
who need to explain are the ones that disable COW, and the reversed
bit would mark that case.
The people who are ok with COW don't need to explain anything, and for
the same reason I think they shouldn't need to add that
FAULT_FLAG_BREAK_COW.
See what my argument is? It's entirely about documenting the special case.
Linus
next prev parent reply other threads:[~2020-08-10 23:19 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-10 14:57 [PATCH v2] mm/gup: Allow real explicit breaking of COW Peter Xu
2020-08-10 16:47 ` Linus Torvalds
2020-08-10 19:15 ` Peter Xu
2020-08-10 20:51 ` Linus Torvalds
2020-08-10 20:59 ` Linus Torvalds
2020-08-10 21:57 ` Peter Xu
2020-08-10 23:19 ` Linus Torvalds [this message]
2020-08-10 23:38 ` Jann Horn
2020-08-11 15:05 ` Linus Torvalds
2020-08-11 15:27 ` Peter Xu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAHk-=whng9zK-GMM+UHsTptF=mgH164Zt_Gok3ohbcWaYee71Q@mail.gmail.com' \
--to=torvalds@linux-foundation.org \
--cc=aarcange@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=hch@lst.de \
--cc=jack@suse.cz \
--cc=jannh@google.com \
--cc=kirill@shutemov.name \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mcfadden8@llnl.gov \
--cc=oleg@redhat.com \
--cc=peterx@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).