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

[Shakeel Butt <shakeel.butt@linux.dev>]

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?
> 
> 

Lorenzo already explained that it is not an issue at the moment. I think
this is good discussion to have as I think we will be expanding the
limit from UIO_FASTIOV to higher value (maybe UIO_MAXIOV) soon in the
followup. At the moment, my gut feeling is that batch size of regions
given to the syscall will depend on the advise parameter. For example,
For MADV_[NO]HUGEPAGE which is a simple flag [re]set, a single write
lock and possible a single tree traversal will be better than multiple
write lock/unlock operations even for large batch size. Anyways we will
need some experimental data to show that.

JFYI SJ is planning to work on two improvements: (1) single tree
traversal for all the given regions and (2) TLB flush batching for
MADV_DONTNEED[_LOCKED] and MADV_FREE.

Thanks everyone for your time and feedback.