linux-kernel - Re: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a pmd

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wh4waroKr-Xtcv+5pTxBcHxGEj-g73eQvXVawML_C0EXw@mail.gmail.com>
Date:   Wed, 9 Oct 2019 09:21:08 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Matthew Wilcox <willy@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Huang Ying <ying.huang@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH v4 3/9] mm: pagewalk: Don't split transhuge pmds when a
 pmd_entry is present

On Wed, Oct 9, 2019 at 8:27 AM Kirill A. Shutemov <kirill@...temov.name> wrote:
>
> Do we have any current user that expect split_huge_pmd() in this scenario.

No. There are no current users of the pmd callback and the pte
callback at all, that I could find.

But it looks like the new drm use does want a "I can't handle the
hugepage, please split it and I'll fo the ptes instead".

> That's hacky.
>
> Maybe just use an error code for this? -EAGAIN?

I actually like the PAGE_WALK_FALLBACK thing as more documentation
than "it's an error, but not one you return", although I do detest the
particular value chosen, which is just a nasty bitpattern.

Maybe it could use an error value, just one that makes no sense, and
is hidden by the PAGE_WALK_FALLBACK define, ie something like

  #define PAGE_WALK_FALLBACK (-ECHILD)

or something like that.

And I suspect the conditional would be cleaner if it was written something like

        if (!err)
                continue;
        if (err != PAGE_WALK_FALLBACK)
                break;
        err = 0;
        if (pmd_trans_unstable(pmd))
                goto again;
        .. do the split ..

and skip the WARN_ON() and the odd "non-zero but smaller than MAX test"

            Linus