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 05:51:45PM +0000, Lorenzo Stoakes wrote:
> 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...!)

Surprised, perhaps because I was wrong about this :) Apologies for that.

SJ raised this in [0] and the non-RFC version of this series is over at [1].

[0]: https://lore.kernel.org/all/20250517162048.36347-1-sj@kernel.org/
[1]: https://lore.kernel.org/all/20250206061517.2958-1-sj@kernel.org/

We should revisit this and determine whether the drop/reacquire lock is
required, perhaps doing some experiments around heavy operations using
UIO_MAXIOV entries?

SJ - could you take a look at this please?

>
> 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.

Again as I said here, I suspect _probably_ this won't be too much of an
issue - but it is absolutely one we need to address.

>
> > Thanks,
> > Liam
> >

Cheers, Lorenzo