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

[Yang Shi]

On Wed, Jun 24, 2020 at 1:23 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Wed, Jun 24, 2020 at 12:21 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > 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.
>
> The __remove_mapping() will try to freeze page count if the count is
> expected otherwise just not discard the page. I'm not quite sure why
> the check is done that late, my wild guess is to check the refcount at
> the last minute so there might be a chance the pin gets released right
> before it.
>
> But I noticed a bug in __remove_ampping() for THP since THP's dma
> pinned count is recorded in the tail page's hpage_pinned_refcount
> instead of refcount. So, the refcount freeze might be successful for
> pinned THP.  Chris's patch could solve this issue too, but I'm not

This bug is not valid. I just realized try_grab_page() would increase
both refcount and hpage_pinned_refcount.

> sure if it is worth backing earlier once dma pinned page is met. If it
> is worth, the follow-up question is why not just skip such page in
> scan phase?
>
> >
> > Jason
> >