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

[Zach O'Keefe]

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,
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);
> >   }
> >
>