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From:   Yang Shi <shy828301@gmail.com>
Date:   Thu, 25 Jun 2020 09:32:34 -0700
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Subject: Re: [PATCH] mm: Skip opportunistic reclaim for dma pinned pages
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Chris Wilson <chris@chris-wilson.co.uk>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        intel-gfx@lists.freedesktop.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>
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On Thu, Jun 25, 2020 at 4:42 AM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Jun 24, 2020 at 08:14:17PM +0100, Chris Wilson wrote:
> > A side effect of the LRU shrinker not being dma aware is that we will
> > often attempt to perform direct reclaim on the persistent group of dma
> > pages while continuing to use the dma HW (an issue as the HW may already
> > be actively waiting for the next user request), and even attempt to
> > reclaim a partially allocated dma object in order to satisfy pinning
> > the next user page for that object.
> >
> > It is to be expected that such pages are made available for reclaim at
> > the end of the dma operation [unpin_user_pages()], and for truly
> > longterm pins to be proactively recovered via device specific shrinkers
> > [i.e. stop the HW, allow the pages to be returned to the system, and
> > then compete again for the memory].
>
> Why are DMA pinned pages still on the LRU list at all?  I never got an
> answer to this that made sense to me.  By definition, a page which is
> pinned for DMA is being accessed, and needs to at the very least change
> position on the LRU list, so just take it off the list when DMA-pinned
> and put it back on the list when DMA-unpinned.

Sounds reasonable to me. In the earlier email I suggested skip isolate
dma pinned page in scan phase, but if they are long term pinned, it
seems preferred to put them on the unevictable lru IMHO.

>
> This overly complex lease stuff must have some reason for existing, but
> I still don't get it.
>