Re: [RFC PATCH v2 4/4] mm/madvise: remove redundant mmap_lock operations from process_madvise()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Jan 31, 2025 at 12:47:24PM -0500, Liam R. Howlett wrote:
> * Davidlohr Bueso <dave@stgolabs.net> [250131 12:31]:
> > On Fri, 31 Jan 2025, Lorenzo Stoakes wrote:
> >
> > > On Thu, Jan 16, 2025 at 05:30:58PM -0800, SeongJae Park wrote:
> > > > Optimize redundant mmap lock operations from process_madvise() by
> > > > directly doing the mmap locking first, and then the remaining works for
> > > > all ranges in the loop.
> > > >
> > > > Signed-off-by: SeongJae Park <sj@kernel.org>
> > >
> > > I wonder if this might increase lock contention because now all of the
> > > vector operations will hold the relevant mm lock without releasing after
> > > each operation?
> >
> > That was exactly my concern. While afaict the numbers presented in v1
> > are quite nice, this is ultimately a micro-benchmark, where no other
> > unrelated threads are impacted by these new hold times.
>
> Indeed, I was also concerned about this scenario.
>
> But this method does have the added advantage of keeping the vma space
> in the same state as it was expected during the initial call - although
> the race does still exist on looking vs acting on the data.  This would
> just remove the intermediate changes.
>
> >
> > > Probably it's ok given limited size of iov, but maybe in future we'd want
> > > to set a limit on the ranges before we drop/reacquire lock?
> >
> > imo, this should best be done in the same patch/series. Maybe extend
> > the benchmark to use IOV_MAX and find a sweet spot?
>
> Are you worried this is over-engineering for a problem that may never be
> an issue, or is there a particular usecase you have in mind?
>
> It is probably worth investigating, and maybe a potential usecase would
> help with the targeted sweet spot?
>

Keep in mind process_madvise() is not limited by IOV_MAX, which can be rather
high, but rather UIO_FASTIOV, which is limited to 8 entries.

(Some have been surprised by this limitation...!)

So I think at this point scaling isn't a huge issue, I raise it because in
future we may want to increase this limit, at which point we should think about
it, which is why I sort of hand-waved it away a bit.

> Thanks,
> Liam
>