Re: Memory allocation on speculative fastpaths

[Matthew Wilcox <willy@infradead.org>]

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:

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:

rcu_read_lock();
vma = vma_lookup();
if (down_read_trylock(&vma->sem)) {
	rcu_read_unlock();
} else {
	rcu_read_unlock();
	mmap_read_lock(mm);
	vma = vma_lookup();
	down_read(&vma->sem);
}

... and we then execute the page table allocation under the protection of
the vma->sem.

At least, that's what I think we agreed to yesterday.