Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

Uladzislau Rezki
2020-08-14T12:15:55Z

> On Thu 13-08-20 19:09:29, Thomas Gleixner wrote:
> > Michal Hocko <mhocko@suse.com> writes:
> [...]
> > > Why should we limit the functionality of the allocator for something
> > > that is not a real problem?
> >
> > We'd limit the allocator for exactly ONE new user which was aware of
> > this problem _before_ the code hit mainline. And that ONE user is
> > prepared to handle the fail.
>
> If we are to limit the functionality to this one particular user then
> I would consider a dedicated gfp flag a huge overkill. It would be much
> more easier to have a preallocated pool of pages and use those and
> completely avoid the core allocator. That would certainly only shift the
> complexity to the caller but if it is expected there would be only that
> single user then it would be probably better than opening a can of worms
> like allocator usable from raw spin locks.
>
Vlastimil raised same question earlier, i answered, but let me answer again:

It is hard to achieve because the logic does not stick to certain static test
case, i.e. it depends on how heavily kfree_rcu(single/double) are used. Based
on that, "how heavily" - number of pages are formed, until the drain/reclaimer
thread frees them.

Preloading pages and keeping them for internal use, IMHO, seems not optimal
from the point of resources wasting. It is better to have a fast mechanism to
request a page and release it back for needs of others. As described about
we do not know how much we will need.

--
Vlad Rezki