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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 14 Aug 2020 15:40:38 -0700
Message-ID: <CAHk-=wifNX6U28sjPay+1ZJ5BmxRG8Bac7W1sP_Ft1yTqLj+GA@mail.gmail.com>
Subject: Re: [PATCH] dma-debug: fix debug_dma_assert_idle(), use rcu_read_lock()
To:     Christoph Hellwig <hch@lst.de>
Cc:     Hugh Dickins <hughd@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Eric Dumazet <edumazet@google.com>,
        iommu <iommu@lists.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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On Thu, Aug 13, 2020 at 10:42 PM Christoph Hellwig <hch@lst.de> wrote:
>
> The whole thing predates my involvement with the code, but I defintively
> think the patch from Hugh is a major improvement.  But I would also
> have no problem with just removing it entirely.

I decided to just do both, since neither you nor Dan seemed to really object.

I applied Hugh's RCU read locking patch as a clear improvement, and
then I did a second patch that just removed this function entirely.
That sounds a bit odd, perhaps, but in case people decide to resurrect
the debugging code, I didn't want us to lose sight of Hugh's
improvement just because I then decided that we might as well go one
step further and just remove it entirely.

And the only real reason I care is that this whole COW and page lock
thing has showed up lately, and I like removing code.

I'm _very_ tempted to just apply my COW simplification patch that gets
rid of all the complex try-to-share cases entirely (and would also
obviate the whole forced-cow patch). I suspect it would effectively
remove almost all of the [un[lock_page() bottlenecks entirely, but
that code has decades of history and I suspect it's a bit too drastic
wrt KSM and the swap cache pages.

It would be lovely if the main source of page locking would really be
about just IO, but the page lock has also become the thing that
serializes almost everything related to page state. Which is why you
find it in contexts that are really not IO-related at all (not just
COW - page migration is the other one that has shown up a lot under
"heavy CPU loads" without really necessarily any IO component to it).

                         Linus