Re: [PATCH v2 2/4] mm/vmalloc: add support for __GFP_NOFAIL

[Michal Hocko]

On Wed 24-11-21 21:37:54, Uladzislau Rezki wrote:
> On Wed, Nov 24, 2021 at 09:43:12AM +0100, Michal Hocko wrote:
> > On Tue 23-11-21 17:02:38, Andrew Morton wrote:
> > > On Tue, 23 Nov 2021 20:01:50 +0100 Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> > > 
> > > > On Mon, Nov 22, 2021 at 04:32:31PM +0100, Michal Hocko wrote:
> > > > > From: Michal Hocko <mhocko@xxxxxxxx>
> > > > > 
> > > > > Dave Chinner has mentioned that some of the xfs code would benefit from
> > > > > kvmalloc support for __GFP_NOFAIL because they have allocations that
> > > > > cannot fail and they do not fit into a single page.
> > > 
> > > Perhaps we should tell xfs "no, do it internally".  Because this is a
> > > rather nasty-looking thing - do we want to encourage other callsites to
> > > start using it?
> > 
> > This is what xfs is likely going to do if we do not provide the
> > functionality. I just do not see why that would be a better outcome
> > though. My longterm experience tells me that whenever we ignore
> > requirements by other subsystems then those requirements materialize in
> > some form in the end. In many cases done either suboptimaly or outright
> > wrong. This might be not the case for xfs as the quality of
> > implementation is high there but this is not the case in general.
> > 
> > Even if people start using vmalloc(GFP_NOFAIL) out of lazyness or for
> > any other stupid reason then what? Is that something we should worry
> > about? Retrying within the allocator doesn't make the things worse. In
> > fact it is just easier to find such abusers by grep which would be more
> > elaborate with custom retry loops.
> >  
> > [...]
> > > > > +		if (nofail) {
> > > > > +			schedule_timeout_uninterruptible(1);
> > > > > +			goto again;
> > > > > +		}
> > > 
> > > The idea behind congestion_wait() is to prevent us from having to
> > > hard-wire delays like this.  congestion_wait(1) would sleep for up to
> > > one millisecond, but will return earlier if reclaim events happened
> > > which make it likely that the caller can now proceed with the
> > > allocation event, successfully.
> > > 
> > > However it turns out that congestion_wait() was quietly broken at the
> > > block level some time ago.  We could perhaps resurrect the concept at
> > > another level - say by releasing congestion_wait() callers if an amount
> > > of memory newly becomes allocatable.  This obviously asks for inclusion
> > > of zone/node/etc info from the congestion_wait() caller.  But that's
> > > just an optimization - if the newly-available memory isn't useful to
> > > the congestion_wait() caller, they just fail the allocation attempts
> > > and wait again.
> > 
> > vmalloc has two potential failure modes. Depleted memory and vmalloc
> > space. So there are two different events to wait for. I do agree that
> > schedule_timeout_uninterruptible is both ugly and very simple but do we
> > really need a much more sophisticated solution at this stage?
> >
> I would say there is at least one more. It is about when users set their
> own range(start:end) where to allocate. In that scenario we might never
> return to a user, because there might not be any free vmap space on
> specified range.
> 
> To address this, we can allow __GFP_NOFAIL only for entire vmalloc
> address space, i.e. within VMALLOC_START:VMALLOC_END.

How should we do that?
-- 
Michal Hocko
SUSE Labs