Re: [QUESTION] about the maple tree and current status of mmap_lock scalability

[Matthew Wilcox <willy@infradead.org>]

On Mon, Feb 20, 2023 at 02:26:49PM +0000, Hyeonggon Yoo wrote:
> On Mon, Jan 02, 2023 at 02:37:02PM +0000, Matthew Wilcox wrote:
> > On Mon, Jan 02, 2023 at 09:04:12PM +0900, Hyeonggon Yoo wrote:
> > > > https://www.infradead.org/~willy/linux/store-free-page-faults.html
> > > > outlines how I intend to proceed from Suren's current scheme (where
> > > > RCU is only used to protect the tree walk) to using RCU for the
> > > > entire page fault.
> > > 
> > > Thank you for sharing this your outlines.
> > > Okay, so the planned scheme is:
> > > 
> > > 	1. Try to process entire page fault under RCU protection
> > > 		- if failed, goto 2. if succeeded, goto 4.
> > > 
> > > 	2. Fall back to Suren's scheme (try to take VMA rwsem)
> > > 		- if failed, goto 3. if succeeded, goto 4.
> > 
> > Right.  The question is whether to restart the page fault under Suren's
> > scheme, or just grab the VMA rwsem and continue.  Experimentation
> > needed.
> > 
> > It's also worth noting that Michel has an alternative proposal, which
> > is to drop out of RCU protection before trying to allocate memory, then
> > re-enter RCU mode and check the sequence count hasn't changed on the
> > entire MM.  His proposal has the advantage of not trying to allocate
> > memory while holding the RCU read lock, but the disadvantage of having
> > to retry the page fault if anyone has called mmap() or munmap().  Which
> > alternative is better is going to depend on the workload; do we see more
> > calls to mmap()/munmap(), or do we need to enter page reclaim more often?
> > I think they're largely equivalent performance-wise in the fast path.
> > Another metric to consider is code complexity; he thinks his method
> > is easier to understand and I think mine is easier.  To be expected,
> > I suppose ;-)
> 
> I'm planning to suggest a cooperative project to my colleagues
> that would involve making __p?d_alloc() take gfp flags.
> 
> Wondering if there was any progress or conclusion made on which
> approach is better for full RCU page faults, or was there another
> solution proposed?
> 
> Asking this because I don't want to waste my time if the approach
> has been abandoned.

Thanks for checking, but nobody's made any progress on this, that I know
of.

(The __p?d_alloc() approach may also be useful to support vmalloc()
with flags that aren't GFP_KERNEL compatible)