linux-kernel - Re: [PATCH v3 1/7] iov_iter: Introduce fault_in_iov

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Message-ID: <CAHk-=wgqOZmRT_gmAS+K9sA7EYCKM9BYzvJMhy1_P6JaaVGvfA@mail.gmail.com>
Date:   Mon, 26 Jul 2021 10:15:31 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        cluster-devel <cluster-devel@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ocfs2-devel@....oracle.com
Subject: Re: [PATCH v3 1/7] iov_iter: Introduce fault_in_iov_iter helper

On Mon, Jul 26, 2021 at 9:33 AM Jan Kara <jack@...e.cz> wrote:
>
> On Fri 23-07-21 22:58:34, Andreas Gruenbacher wrote:
> > +     gup_flags = FOLL_TOUCH | FOLL_POPULATE;
>
> I don't think FOLL_POPULATE makes sense here. It makes sense only with
> FOLL_MLOCK and determines whether mlock(2) should fault in missing pages or
> not.

Yeah, it won't hurt, but FOLL_POPULATE doesn't actually do anything
unless FOLL_MLOCK is set. It is, as you say, a magic flag just for
mlock.

The only ones that should matter are FOLL_WRITE (for obvious reasons)
and FOLL_TOUCH (to set the accessed and dirty bits, rather than just
th protection bits)

                   Linus