Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures, honor failures

[Pavel Tatashin]

On Fri, Dec 18, 2020 at 8:14 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> On Fri 18-12-20 07:43:15, Pavel Tatashin wrote:
> > On Fri, Dec 18, 2020 at 5:46 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > >
> > > On Thu 17-12-20 13:52:41, Pavel Tatashin wrote:
> > > [...]
> > > > +#define PINNABLE_MIGRATE_MAX 10
> > > > +#define PINNABLE_ISOLATE_MAX 100
> > >
> > > Why would we need to limit the isolation retries. Those should always be
> > > temporary failure unless I am missing something.
> >
> > Actually, during development, I was retrying isolate errors
> > infinitely, but during testing found a hung where when FOLL_TOUCH
> > without FOLL_WRITE is passed (fault in kernel without write flag), the
> > zero page is faulted. The isolation of the zero page was failing every
> > time, therefore the process was hanging.
>
> Why would you migrate zero page in the first place? Simply instantiate
> it.

This is exactly the idea behind FOLL_WRITE; it causes zero pages to be
created in the right zone right away, and no migration is necessary.

>
> > Since then, I fixed this problem by adding FOLL_WRITE unconditionally
> > to FOLL_LONGTERM, but I was worried about other possible bugs that
> > would cause hangs, so decided to limit isolation errors. If you think
> > it its not necessary, I can unlimit isolate retires.
>
> It should have a really good reason to exist. Worries about some corner
> cases is definitely not a reason to put some awkward retry mechanism.
> My historical experience is that these things are extremely hard to get
> rid of later.
>
> > > I am not sure about the
> > > PINNABLE_MIGRATE_MAX either. Why do we want to limit that? migrate_pages
> > > already implements its retry logic why do you want to count retries on
> > > top of that? I do agree that the existing logic is suboptimal because
> >
> > True, but again, just recently, I worked on a race bug where pages can
> > end up in per-cpu list after lru_add_drain_all() but before isolation,
> > so I think retry is necessary.
>
> There are ways to make sure pages are not ending on pcp list. Have a
> look at how hotplug does that.

Sounds good to me, I will remove PINNABLE_MIGRATE_MAX, and leave only
PINNABLE_ISOLATE_MAX for transient isolation errors.

>
> > > the migration failure might be ephemeral or permanent but that should be
> > > IMHO addressed at migrate_pages (resp. unmap_and_move) and simply report
> > > failures that are permanent - e.g. any potential pre-existing long term
> > > pin - if that is possible at all. If not what would cause permanent
> > > migration failure? OOM?
> >
> > Yes, OOM is the main cause for migration failures.
>
> Then you can treat ENOMEM as a permanent failure.

Sounds good.