Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

[Michal Hocko <mhocko@suse.com>]

On Tue 26-10-21 16:25:07, Uladzislau Rezki wrote:
> On Tue, Oct 26, 2021 at 12:24 PM NeilBrown <neilb@suse.de> wrote:
> >
> > On Tue, 26 Oct 2021, Michal Hocko wrote:
> > > On Tue 26-10-21 10:50:21, Neil Brown wrote:
> > > > On Mon, 25 Oct 2021, Uladzislau Rezki wrote:
> > > > > On Fri, Oct 22, 2021 at 09:49:08AM +1100, NeilBrown wrote:
> > > > > > However I'm not 100% certain, and the behaviour might change in the
> > > > > > future.  So having one place (the definition of memalloc_retry_wait())
> > > > > > where we can change the sleeping behaviour if the alloc_page behavour
> > > > > > changes, would be ideal.  Maybe memalloc_retry_wait() could take a
> > > > > > gfpflags arg.
> > > > > >
> > > > > At sleeping is required for __get_vm_area_node() because in case of lack
> > > > > of vmap space it will end up in tight loop without sleeping what is
> > > > > really bad.
> > > > >
> > > > So vmalloc() has two failure modes.  alloc_page() failure and
> > > > __alloc_vmap_area() failure.  The caller cannot tell which...
> > > >
> > > > Actually, they can.  If we pass __GFP_NOFAIL to vmalloc(), and it fails,
> > > > then it must have been __alloc_vmap_area() which failed.
> > > > What do we do in that case?
> > > > Can we add a waitq which gets a wakeup when __purge_vmap_area_lazy()
> > > > finishes?
> > > > If we use the spinlock from that waitq in place of free_vmap_area_lock,
> > > > then the wakeup would be nearly free if no-one was waiting, and worth
> > > > while if someone was waiting.
> > >
> > > Is this really required to be part of the initial support?
> >
> > No.... I was just thinking out-loud.
> >
> alloc_vmap_area() has an retry path, basically if it fails the code
> will try to "purge"
> areas and repeat it one more time. So we do not need to purge outside some where
> else.

I think that Neil was not concerned about the need for purging something
but rather a waiting event the retry loop could hook into. So that the
sleep wouldn't have to be a random timeout but something that is
actually actionable - like somebody freeing an area.
-- 
Michal Hocko
SUSE Labs