Re: [PATCH v2] mm/hugetlb: avoid looping to the same hugepage if !pages and !vmas

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Mike Kravetz <mike.kravetz@xxxxxxxxxx> ä2019å9æ4æåä äå5:26åéï
>
> On 8/29/19 6:50 AM, Zhigang Lu wrote:
> > From: Zhigang Lu <tonnylu@xxxxxxxxxxx>
> >
> > When mmapping an existing hugetlbfs file with MAP_POPULATE, we find
> > it is very time consuming. For example, mmapping a 128GB file takes
> > about 50 milliseconds. Sampling with perfevent shows it spends 99%
> > time in the same_page loop in follow_hugetlb_page().
> >
> > samples: 205  of event 'cycles', Event count (approx.): 136686374
> > -  99.04%  test_mmap_huget  [kernel.kallsyms]  [k] follow_hugetlb_page
> >         follow_hugetlb_page
> >         __get_user_pages
> >         __mlock_vma_pages_range
> >         __mm_populate
> >         vm_mmap_pgoff
> >         sys_mmap_pgoff
> >         sys_mmap
> >         system_call_fastpath
> >         __mmap64
> >
> > follow_hugetlb_page() is called with pages=NULL and vmas=NULL, so for
> > each hugepage, we run into the same_page loop for pages_per_huge_page()
> > times, but doing nothing. With this change, it takes less then 1
> > millisecond to mmap a 128GB file in hugetlbfs.
>
> Thanks for the analysis!
>
> Just curious, do you have an application that does this (mmap(MAP_POPULATE)
> for an existing hugetlbfs file), or was this part of some test suite or
> debug code?

Yes, DPDK and SPDK actually do this in vhost_user.c.
vhost_setup_mem_table() {
...
mmap_size = RTE_ALIGN_CEIL(mmap_size, alignment);

mmap_addr = mmap(NULL, mmap_size, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_POPULATE, fd, 0);
...
}

>
> > Signed-off-by: Zhigang Lu <tonnylu@xxxxxxxxxxx>
> > Reviewed-by: Haozhong Zhang <hzhongzhang@xxxxxxxxxxx>
> > Reviewed-by: Zongming Zhang <knightzhang@xxxxxxxxxxx>
> > Acked-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > ---
> >  mm/hugetlb.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 6d7296d..2df941a 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -4391,6 +4391,17 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                               break;
> >                       }
> >               }
>
> It might be helpful to add a comment here to help readers of the code.
> Something like:
>

Thanks, I will add this comment and send a new version.

>                 /*
>                  * If subpage information not requested, update counters
>                  * and skip the same_page loop below.
>                  */
> > +
> > +             if (!pages && !vmas && !pfn_offset &&
> > +                 (vaddr + huge_page_size(h) < vma->vm_end) &&
> > +                 (remainder >= pages_per_huge_page(h))) {
> > +                     vaddr += huge_page_size(h);
> > +                     remainder -= pages_per_huge_page(h);
> > +                     i += pages_per_huge_page(h);
> > +                     spin_unlock(ptl);
> > +                     continue;
> > +             }
> > +
> >  same_page:
> >               if (pages) {
> >                       pages[i] = mem_map_offset(page, pfn_offset);
> >
>
> With a comment added to the code,
> Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> --
> Mike Kravetz