Re: Memory allocation on speculative fastpaths

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, May 03, 2022 at 04:15:46PM -0700, Suren Baghdasaryan wrote:
> On Tue, May 3, 2022 at 11:28 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Tue, May 03, 2022 at 09:39:05AM -0700, Paul E. McKenney wrote:
> > > On Tue, May 03, 2022 at 06:04:13PM +0200, Michal Hocko wrote:
> > > > On Tue 03-05-22 08:59:13, Paul E. McKenney wrote:
> > > > > Hello!
> > > > >
> > > > > Just following up from off-list discussions yesterday.
> > > > >
> > > > > The requirements to allocate on an RCU-protected speculative fastpath
> > > > > seem to be as follows:
> > > > >
> > > > > 1.        Never sleep.
> > > > > 2.        Never reclaim.
> > > > > 3.        Leave emergency pools alone.
> > > > >
> > > > > Any others?
> > > > >
> > > > > If those rules suffice, and if my understanding of the GFP flags is
> > > > > correct (ha!!!), then the following GFP flags should cover this:
> > > > >
> > > > >   __GFP_NOMEMALLOC | __GFP_NOWARN
> > > >
> > > > GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN
> > >
> > > Ah, good point on GFP_NOWAIT, thank you!
> >
> > Johannes (I think it was?) made the point to me that if we have another
> > task very slowly freeing memory, a task in this path can take advantage
> > of that other task's hard work and never go into reclaim.  So the
> > approach we should take is:

Right, GFP_NOWAIT can starve out other allocations. It can clear out
the freelists without the burden of having to do reclaim like
everybody else wanting memory during a shortage. Including GFP_KERNEL.

In smaller doses and/or for privileged purposes (e.g. single-argument
kfree_rcu ;)), those allocations are fine. But because the context is
page tables specifically, it would mean that userspace could trigger a
large number of those and DOS other applications and the kernel.

> > p4d_alloc(GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);
> > pud_alloc(GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);
> > pmd_alloc(GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);
> >
> > if (failure) {
> >   rcu_read_unlock();
> >   do_reclaim();
> >   return FAULT_FLAG_RETRY;
> > }
> >
> > ... but all this is now moot since the approach we agreed to yesterday
> > is:
> 
> I think the discussion was about the above approach and Johannes
> suggested to fallback to the normal pagefault handling with mmap_lock
> locked if PMD does not exist. Please correct me if I misunderstood
> here.

Yeah. Either way works, as long as the task is held accountable.