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Date:   Mon, 14 Sep 2020 14:34:36 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jason Gunthorpe <jgg@nvidia.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Maya B . Gokhale" <gokhale2@llnl.gov>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Marty Mcfadden <mcfadden8@llnl.gov>,
        Kirill Shutemov <kirill@shutemov.name>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Message-ID: <20200914183436.GD30881@xz-x1>
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On Mon, Sep 14, 2020 at 10:32:11AM -0700, Linus Torvalds wrote:
> On Mon, Sep 14, 2020 at 7:38 AM Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > I don't have a detailed explanation right now, but this patch appears
> > to be causing a regression where RDMA subsystem tests fail. Tests
> > return to normal when this patch is reverted.
> >
> > It kind of looks like the process is not seeing DMA'd data to a
> > pin_user_pages()?
> 
> I'm a nincompoop. I actually _talked_ to Hugh Dickins about this when
> he raised concerns, and I dismissed his concerns with "but PAGE_PIN is
> special".
> 
> As usual, Hugh was right. Page pinning certainly _is_ special, but
> it's not that different from the regular GUP code.
> 
> But in the meantime, I have a lovely confirmation from the kernel test
> robot, saying that commit 09854ba94c results in a
> "vm-scalability.throughput 31.4% improvement", which was what I was
> hoping for - the complexity wasn't just complexity, it was active
> badness due to the page locking horrors.
> 
> I think what we want to do is basically do the "early COW", but only
> do it for FOLL_PIN (and not turn them into writes for anything but the
> COW code). So basically redo the "enforced COW mechanism", but rather
> than do it for everything, now do it only for FOLL_PIN, and only in
> that COW path.
> 
> Peter - any chance you can look at this? I'm still looking at the page
> lock fairness performance regression, although I now think I have a
> test patch for Phoronix to test out.

Sure, I'll try to prepare something like that and share it shortly.

Thanks,

-- 
Peter Xu