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

[Jason Gunthorpe]

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