Re: [PATCH -next] mm: usercopy: add a debugfs interface to bypass the vmalloc check.

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Dec 04, 2024 at 05:21:12PM +0800, zuoze wrote:
> 
> 
> 在 2024/12/4 15:55, Uladzislau Rezki 写道:
> > On Tue, Dec 03, 2024 at 07:56:34PM +0000, Matthew Wilcox wrote:
> > > On Tue, Dec 03, 2024 at 08:02:26PM +0100, Uladzislau Rezki wrote:
> > > 
> > > I think there are a few other things we can try here.
> > > 
> > > First, if the copy is small (and I still don't have an answer to that
> > > ...), we can skip the vmalloc lookup if the copy doesn't cross a page
> > > boundary.
> > > 
> > > Second, we could try storing this in a maple tree rather than an rbtree.
> > > That gives us RCU protected lookups rather than under a spinlock.
> > > 
> > > It might even be worth going to a rwlock first, in case the problem is
> > > that there's severe lock contention.
> > > 
> > > But I've asked for data on spinlock contention and not received an
> > > answer on that either, so I don't know what to suggest.
> > > 
> > I think, it is not about contention. It is about the extra "attached
> > load" when a data is heavily copied force and back. On each copy path
> > you need to do a scan. Maple tree is not that something can help here :)
> > 
> > Indeed, no contention data. Zuoze, please share this if you can.
> 
> We have enabled perf lock contention and are currently debugging the
> environment. We will share the results as soon as we have them.
> 
Sounds good and thank you for helping to improve this :)

--
Uladzislau Rezki