Re: [PATCH 2/2] mm: mmap: map MAP_STACK to VM_NOHUGEPAGE

[Yang Shi]

On Tue, Jan 16, 2024 at 11:22 AM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
>
> Thanks Yang,
>
> Should this be marked for stable? Given how easily it is for pthreads
> to allocate hugepages w/o this change, it can easily cause memory
> bloat on larger systems and/or users with high thread counts. I don't
> think that will be welcomed, and seems odd that just 6.7 should suffer
> this.

Thanks for the suggestion, fine to me.

>
> Thanks,
> Zach
>
> On Tue, Jan 9, 2024 at 5:36 PM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
> >
> >
> >
> > On 2023/12/21 14:59, Yang Shi wrote:
> > > From: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
> > >
> > > The commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
> > > boundaries") incured regression for stress-ng pthread benchmark [1].
> > > It is because THP get allocated to pthread's stack area much more possible
> > > than before.  Pthread's stack area is allocated by mmap without VM_GROWSDOWN
> > > or VM_GROWSUP flag, so kernel can't tell whether it is a stack area or not.
> > >
> > > The MAP_STACK flag is used to mark the stack area, but it is a no-op on
> > > Linux.  Mapping MAP_STACK to VM_NOHUGEPAGE to prevent from allocating
> > > THP for such stack area.
> > >
> > > With this change the stack area looks like:
> > >
> > > fffd18e10000-fffd19610000 rw-p 00000000 00:00 0
> > > Size:               8192 kB
> > > KernelPageSize:        4 kB
> > > MMUPageSize:           4 kB
> > > Rss:                  12 kB
> > > Pss:                  12 kB
> > > Pss_Dirty:            12 kB
> > > Shared_Clean:          0 kB
> > > Shared_Dirty:          0 kB
> > > Private_Clean:         0 kB
> > > Private_Dirty:        12 kB
> > > Referenced:           12 kB
> > > Anonymous:            12 kB
> > > KSM:                   0 kB
> > > LazyFree:              0 kB
> > > AnonHugePages:         0 kB
> > > ShmemPmdMapped:        0 kB
> > > FilePmdMapped:         0 kB
> > > Shared_Hugetlb:        0 kB
> > > Private_Hugetlb:       0 kB
> > > Swap:                  0 kB
> > > SwapPss:               0 kB
> > > Locked:                0 kB
> > > THPeligible:           0
> > > VmFlags: rd wr mr mw me ac nh
> > >
> > > The "nh" flag is set.
> > >
> > > [1] https://lore.kernel.org/linux-mm/202312192310.56367035-oliver.sang@xxxxxxxxx/
> > >
> > > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> > > Tested-by: Oliver Sang <oliver.sang@xxxxxxxxx>
> > > Cc: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> > > Cc: Rik van Riel <riel@xxxxxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: Christopher Lameter <cl@xxxxxxxxx>
> > > Cc: Huang, Ying <ying.huang@xxxxxxxxx>
> > > Signed-off-by: Yang Shi <yang@xxxxxxxxxxxxxxxxxxxxxx>
> >
> > Reviewed-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
> >
> > > ---
> > >   include/linux/mman.h | 1 +
> > >   1 file changed, 1 insertion(+)
> > >
> > > diff --git a/include/linux/mman.h b/include/linux/mman.h
> > > index 40d94411d492..dc7048824be8 100644
> > > --- a/include/linux/mman.h
> > > +++ b/include/linux/mman.h
> > > @@ -156,6 +156,7 @@ calc_vm_flag_bits(unsigned long flags)
> > >       return _calc_vm_trans(flags, MAP_GROWSDOWN,  VM_GROWSDOWN ) |
> > >              _calc_vm_trans(flags, MAP_LOCKED,     VM_LOCKED    ) |
> > >              _calc_vm_trans(flags, MAP_SYNC,       VM_SYNC      ) |
> > > +            _calc_vm_trans(flags, MAP_STACK,      VM_NOHUGEPAGE) |
> > >              arch_calc_vm_flag_bits(flags);
> > >   }
> > >
> >