Date:   Wed, 22 Feb 2023 11:38:37 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     linux-mm@kvack.org, liam.howlett@oracle.com, surenb@google.com,
        ldufour@linux.ibm.com, michel@lespinasse.org, vbabka@suse.cz,
        linux-kernel@vger.kernel.org
Subject: Re: [QUESTION] about the maple tree and current status of mmap_lock
 scalability
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On Mon, Feb 20, 2023 at 02:43:23PM +0000, Matthew Wilcox wrote:
> 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.

Thanks for confirmation. then I think it's still worth trying.

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

Is there any possible users of that,
sounds like someone tries to call __vmalloc() in interrupt context or
RCU read-side critical section?