Re: [RFC PATCH 01/31] mm: migrate: Add exchange_pages to exchange two lists of pages.

[Matthew Wilcox]

On Mon, Feb 18, 2019 at 09:51:33AM -0800, Zi Yan wrote:
> On 18 Feb 2019, at 9:42, Vlastimil Babka wrote:
> > On 2/18/19 6:31 PM, Zi Yan wrote:
> > > The purpose of proposing exchange_pages() is to avoid allocating any
> > > new
> > > page,
> > > so that we would not trigger any potential page reclaim or memory
> > > compaction.
> > > Allocating a temporary page defeats the purpose.
> > 
> > Compaction can only happen for order > 0 temporary pages. Even if you
> > used
> > single order = 0 page to gradually exchange e.g. a THP, it should be
> > better than
> > u64. Allocating order = 0 should be a non-issue. If it's an issue, then
> > the
> > system is in a bad state and physically contiguous layout is a secondary
> > concern.
> 
> You are right if we only need to allocate one order-0 page. But this also
> means
> we can only exchange two pages at a time. We need to add a lock to make sure
> the temporary page is used exclusively or we need to keep allocating
> temporary pages
> when multiple exchange_pages() are happening at the same time.

You allocate one temporary page per thread that's doing an exchange_page().