linux-kernel - Re: [PATCH 07/25] mm/csky: Use mm_fault

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Date:   Wed, 17 Jun 2020 13:15:31 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Guo Ren <guoren@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-csky@...r.kernel.org
Subject: Re: [PATCH 07/25] mm/csky: Use mm_fault_accounting()

On Wed, Jun 17, 2020 at 12:58 PM Peter Xu <peterx@...hat.com> wrote:
>
> But currently remote GUP will still do the page fault accounting on the remote
> task_struct, am I right?  E.g., when the get_user_pages_remote() is called with
> "tsk != current", it seems the faultin_page() will still do maj_flt/min_flt
> accounting for that remote task/thread?

Well, that would be a data race and fundamentally buggy.

It would be ok with something like ptrace (which only works when the
target is quiescent), but is completely wrong otherwise.

I guess it works fine in practice, and it's only statistics so even if
you were to have a data race it doesn't much matter, but it's
definitely conceptually very very wrong.

The fault stats should be about who does the fault (they are about the
_thread_) not about who the fault is done to (which is about the
_mm_).

Allocating the fault data to somebody else sounds frankly silly and
stupid to me, exactly because it's (a) racy and (b) not even
conceptually correct. The other thread literally _isn't_ doing a major
page fault, for crissake!

Now, there are some actual per-mm statistics too (the rss stuff etc),
and it's fundamentally harder exactly because of the shared data. See
the mm_counter stuff etc. Those are not about who does soemthing, they
are about the resulting MM state.

            Linus