linux-kernel - Re: Access to non-RAM pages

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Date:   Sun, 2 Sep 2018 19:52:02 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Jürgen Groß <jgross@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Will Deacon <will.deacon@....com>
Subject: Re: Access to non-RAM pages

On Sun, Sep 2, 2018 at 7:47 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The comment actually does talk about it, although the comment also
> claims that the cs read would use load_unaligned_zeropad(), which it
> no longer does (now it only does the read_word_at_a_time).

IOW, look at commit 12f8ad4b0533 ("vfs: clean up __d_lookup_rcu() and
dentry_cmp() interfaces") for why the zeropad went away for the cs
access (but the comment wasn't updated).

And then bfe7aa6c39b1 ("fs/dcache: Use read_word_at_a_time() in
dentry_string_cmp()") did the "let's make KASAN happy thing.

And yes, the word-at-a-time code actually matters a lot for certain
loads. The "copy-and-hash" thing for path components ends up being
pretty critical in all the pathname handling.

            Linus