linux-kernel - Re: [PATCH 1/4] mm: Trial do_wp

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Message-ID: <20200917220900.GO8409@ziepe.ca>
Date:   Thu, 17 Sep 2020 19:09:00 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

On Thu, Sep 17, 2020 at 05:40:59PM -0400, Peter Xu wrote:
> On Thu, Sep 17, 2020 at 01:35:56PM -0700, Linus Torvalds wrote:
> > For that to happen, we'd need to have the vma flag so that we wouldn't
> > have any worry about non-pinners, but as you suggested, I think even
> > just a mm-wide counter - or flag - to deal with the fast-bup case is
> > likely perfectly sufficient.
> 
> Would mm_struct.pinned_vm suffice?

I think that could be a good long term goal

IIRC last time we dug into the locked_vm vs pinned_vm mess it didn't
get fixed. There is a mix of both kinds, as you saw, and some
resistance I don't clearly remember to changing it.

My advice for this -rc fix is to go with a single bit in the mm_struct
set on any call to pin_user_pages*

Then only users using pin_user_pages and forking are the only ones who
would ever do extra COW on fork. I think that is OK for -rc, this
workload should be rare due to the various historical issues. Anyhow,
a slow down regression is better than a it is broken regression.

This can be improved into a counter later. Due to the pinned_vm
accounting all call sites should have the mm_struct at unpin, but I
have a feeling it will take a alot of driver patches to sort it all
out.

Jason