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SubjectRe: Memory allocation on speculative fastpaths
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!

> > Or is this just a fancy way of always returning NULL or some such? ;-)
>
> It could fail quite easily. We would also want to guarantee (by
> documenting I guess) that the page allocator never does anything that
> would depend or invoke rcu_synchronize or something like that.

The GPF_NOWAIT should rule out synchronize_rcu() and similar, correct?

> I believe this is the case currently.

Here is hoping! ;-)

Thanx, Paul

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