linux-kernel - Re: [PATCH] mm: Skip opportunistic reclaim for dma pinned pages

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Message-ID: <20200625160522.GD7703@casper.infradead.org>
Date:   Thu, 25 Jun 2020 17:05:22 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     Chris Wilson <chris@...is-wilson.co.uk>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH] mm: Skip opportunistic reclaim for dma pinned pages

On Thu, Jun 25, 2020 at 03:40:44PM +0200, Jan Kara wrote:
> On Thu 25-06-20 12:42:09, Matthew Wilcox wrote:
> > 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.
> 
> Well, we do mark_page_accessed() when pinning in GUP. This is not perfect
> but it's as good as it gets with CPU having no control when the page is
> actually accessed. Also taking the page off and then back to LRU list would
> increase the contention on the LRU list locks and generally cost
> performance so for short term pins it is not desirable... Otherwise I agree
> that conceptually it would make some sence although I'm not sure some
> places wouldn't get confused by e.g. page cache pages not being on LRU
> list. 

We could/should do what we do for mlocked pages:
Documentation/vm/unevictable-lru.rst

I think 'case five' is wrong and needs to be removed.  Pinning is
inappropriate for "I'm going to modify the page myself".