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

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Message-ID: <20200917202514.GN8409@ziepe.ca>
Date:   Thu, 17 Sep 2020 17:25:14 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Xu <peterx@...hat.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 01:19:02PM -0700, John Hubbard wrote:
> On 9/17/20 1:06 PM, Jason Gunthorpe wrote:
> > On Thu, Sep 17, 2020 at 12:42:11PM -0700, Linus Torvalds wrote:
> ...
> > > Is there possibly somethign else we can filter on than just
> > > GUP_PIN_COUNTING_BIAS? Because it could be as simple as just marking
> > > the vma itself and saying "this vma has had a page pinning event done
> > > on it".
> > 
> > We'd have to give up pin_user_pages_fast() to do that as we can't fast
> > walk and get vmas?
> 
> oops, yes. I'd forgotten about that point. Basically all of the O_DIRECT
> callers need _fast. This is a big problem.

What about an atomic counter in the mm_struct? 

# of pages currently under pin.

The use case Linus is worried about will never have any pins, so it
would be 0 and it could skip this entire test.

Jason