linux-kernel - Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

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Date:   Thu, 23 Sep 2021 13:53:28 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH] mm/khugepaged: Detecting uffd-wp vma more efficiently

On Thu, Sep 23, 2021 at 09:47:42AM -0700, Axel Rasmussen wrote:
> My thinking there was a THP collapse bug was really just based on
> speculation, not a real reproducer, so it's very possible my
> speculation was wrong. It will take some more thinking and reading to
> convince myself one way or the other. :) Thanks to you and Hugh for
> all the details.
> 
> I'd prefer not to add this fix "just in case", if it isn't a real
> problem, as it seems like it may confuse future readers of the code.

It's not "just in case" to me - IMHO it's theoretically causing more false
positives as I used to mention, at least that's my understanding so far. So if
the theory is correct it'll 100% happen when khugepaged merged some
minor-registered regions.

Uffd-wp could have many false positives like this if we don't support swap - at
last we decided to fully support swap then we removed all the false positives
regarding swapping.  I think it's similar here, but khugepaged should trigger
much less frequently on the false positives upon uffd-minor, than swapping upon
uffd-wp.

But yes, there's definitely no rush on thinking or anything - it'll never hurt
to think more. And more importantly, verify it with some test program would be
great; after all theoretically it'll just work like a charm to me.

> 
> I'll send out a patch for it if / when I manage to build a real
> reproducer. Or, in the meantime, some of my Google colleagues are
> testing this code via their live migration implementation, so if there
> is a bug here there's a good chance we'll find it that way too.

Sounds like a good plan.

Thanks,

-- 
Peter Xu