On Fri, Jan 31, 2025 at 12:47:24PM -0500, Liam R. Howlett wrote: > * Davidlohr Bueso <dave@xxxxxxxxxxxx> [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@xxxxxxxxxx> > > > > > > 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 >
