On Wed 01-03-17 13:31:49, Johannes Weiner wrote: > On Wed, Mar 01, 2017 at 02:36:24PM +0100, Michal Hocko wrote: > > On Fri 24-02-17 13:31:49, Shaohua Li wrote: > > > show MADV_FREE pages info of each vma in smaps. The interface is for > > > diganose or monitoring purpose, userspace could use it to understand > > > what happens in the application. Since userspace could dirty MADV_FREE > > > pages without notice from kernel, this interface is the only place we > > > can get accurate accounting info about MADV_FREE pages. > > > > I have just got to test this patchset and noticed something that was a > > bit surprising > > > > madvise(mmap(len), len, MADV_FREE) > > Size: 102400 kB > > Rss: 102400 kB > > Pss: 102400 kB > > Shared_Clean: 0 kB > > Shared_Dirty: 0 kB > > Private_Clean: 102400 kB > > Private_Dirty: 0 kB > > Referenced: 0 kB > > Anonymous: 102400 kB > > LazyFree: 102368 kB > > > > It took me a some time to realize that LazyFree is not accurate because > > there are still pages on the per-cpu lru_lazyfree_pvecs. I believe this > > is an implementation detail which shouldn't be visible to the userspace. > > Should we simply drain the pagevec? A crude way would be to simply > > lru_add_drain_all after we are done with the given range. We can also > > make this lru_lazyfree_pvecs specific but I am not sure this is worth > > the additional code. > > --- > > diff --git a/mm/madvise.c b/mm/madvise.c > > index dc5927c812d3..d2c318db16c9 100644 > > --- a/mm/madvise.c > > +++ b/mm/madvise.c > > @@ -474,7 +474,7 @@ static int madvise_free_single_vma(struct vm_area_struct *vma, > > madvise_free_page_range(&tlb, vma, start, end); > > mmu_notifier_invalidate_range_end(mm, start, end); > > tlb_finish_mmu(&tlb, start, end); > > - > > + lru_add_drain_all(); > > A full drain on all CPUs is very expensive and IMO not justified for > some per-cpu fuzz factor in the stats. I'd take hampering the stats > over hampering the syscall any day; only a subset of MADV_FREE users > will look at the stats. > > And while the aggregate error can be large on machines with many CPUs > (notably the machines on which you absolutely don't want to send IPIs > to all cores each time a thread madvises some pages!), I am not sure I understand. Where would we trigger IPIs? lru_add_drain_all relies on workqueus. > the pages of a > single process are not likely to be spread out across more than a few > CPUs. Then we can simply only flushe lru_lazyfree_pvecs which should reduce the unrelated noise from other pagevecs. > The error when reading a specific smaps should be completely ok. > > In numbers: even if your process is madvising from 16 different CPUs, > the error in its smaps file will peak at 896K in the worst case. That > level of concurrency tends to come with much bigger memory quantities > for that amount of error to matter. It is still an unexpected behavior IMHO and an implementation detail which leaks to the userspace. > IMO this is a non-issue. I will not insist if there is a general consensus on this and it is a documented behavior, though. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
