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

[Yang Shi <shy828301@gmail.com>]

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.
>