linux-kernel - Re: [PATCH 1/5] mm: Introduce mm_struct.has

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Message-ID: <20200926004136.GJ9916@ziepe.ca>
Date:   Fri, 25 Sep 2020 21:41:36 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Peter Xu <peterx@...hat.com>, John Hubbard <jhubbard@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Michal Hocko <mhocko@...e.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Christoph Hellwig <hch@....de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Jann Horn <jannh@...gle.com>
Subject: Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

On Fri, Sep 25, 2020 at 02:06:59PM -0700, Linus Torvalds wrote:
> On Fri, Sep 25, 2020 at 12:56 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > And honestly, since this is all getting fairly late in the rc, and it
> > took longer than I thought, I think we should do the GFP_ATOMIC
> > approach for now - not great, but since it only triggers for this case
> > that really should never happen anyway, I think it's probably the best
> > thing for 5.9, and we can improve on things later.
> 
> I'm not super-happy with this patch, but I'm throwing it out anyway, in case
> 
>  (a) somebody can test it - I don't have any test cases

It looks like it will work and resolve the RDMA case that triggered
this discussion. I will send it to our testers, should hear back
around Monday.

They previously said Peter's v1 patch worked, expecting the same here,
unless something unexpected hits the extra pre-conditions.

Though, we might hit the THP case and find it fails...

>  (b) somebody can find issues and improve on it

The THP hunks from Peter's series looked pretty straightforward, I'd
include at least the PMD one.

As a tiny optimization, the preconditions in copy_normal_page() could
order the atomics last to try and reduce the atomics done per fork.

> I'm happy to take Peter's series too, this is more of an alternative
> simplified version to keep the discussion going.

I don't completely grok the consequences of the anon_vma check. We
can exclude file backed mappings as they are broken for pinning
anyhow, so what is left that could be MAP_PRIVATE of a non-anon_vma?

Feels obscure, probably OK. If something does break userspace could
use MAP_SHARED and be fixed.

Otherwise, I do prefer Peter's version because of the GFP_KERNEL. To
touch on your other email..

It was my hope we could move away from the "This should never
happen". From a RDMA POV this idea was sort of managable years ago,
but now I have folks writing data science/ML software in Python that
deep under the libraries use RDMA and has pinned pages. It was a
Python program that detected this regression.

Having all that "just work" regardless of what foolish stuff happens
in the Python layer is very appealing.

Jason