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

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Message-ID: <20200624192116.GO6578@ziepe.ca>
Date:   Wed, 24 Jun 2020 16:21:16 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH] mm: Skip opportunistic reclaim for dma pinned pages

On Wed, Jun 24, 2020 at 08:14:17PM +0100, Chris Wilson wrote:
> A general rule of thumb is that shrinkers should be fast and effective.
> They are called from direct reclaim at the most incovenient of times when
> the caller is waiting for a page. If we attempt to reclaim a page being
> pinned for active dma [pin_user_pages()], we will incur far greater
> latency than a normal anonymous page mapped multiple times. Worse the
> page may be in use indefinitely by the HW and unable to be reclaimed
> in a timely manner.

A pinned page can't be migrated, discarded or swapped by definition -
it would cause data corruption.

So, how do things even get here and/or work today at all? I think the
explanation is missing something important.

Jason