Re: [PATCHi v2] mm: put_and_wait_on_page_locked() while page is migrated

[Michal Hocko]

On Tue 27-11-18 16:49:47, Cristopher Lameter wrote:
> On Tue, 27 Nov 2018, Mike Rapoport wrote:
> 
> > >  * @page: The page to wait for.
> > >  *
> > >  * The caller should hold a reference on @page.  They expect the page to
> > >  * become unlocked relatively soon, but do not wish to hold up migration
> > >  * (for example) by holding the reference while waiting for the page to
> > >  * come unlocked.  After this function returns, the caller should not
> > >  * dereference @page.
> > >  */
> >
> > How about:
> >
> > They expect the page to become unlocked relatively soon, but they can wait
> > for the page to come unlocked without holding the reference, to allow
> > other users of the @page (for example migration) to continue.
> 
> All of this seems a bit strange and it seems unnecessary? Maybe we need a
> better explanation?
> 
> A process has no refcount on a page struct and is waiting for it to become
> unlocked? Why? Should it not simply ignore that page and continue? It
> cannot possibly do anything with the page since it does not hold a
> refcount.

So do you suggest busy waiting on the page under migration?
-- 
Michal Hocko
SUSE Labs