Re: [PATCH v3 5/5] powerpc/mm: Fix growth direction for hugepages mmaps with slice

[Christophe LEROY]

Le 24/01/2018 Ã 11:08, Aneesh Kumar K.V a ÃcritÂ:
> On 01/24/2018 03:33 PM, Christophe LEROY wrote:
> > Le 24/01/2018 Ã 10:51, Aneesh Kumar K.V a ÃcritÂ:
> > > On 01/24/2018 03:09 PM, Christophe LEROY wrote:
> > > > Le 24/01/2018 Ã 10:35, Aneesh Kumar K.V a ÃcritÂ:
> > > > >
> > >
> > > > > > Did you try with HUGETLB_MORECORE_HEAPBASE=0x11000000 on PPC64 as 
> > > > > > I suggested in my last email on this subject (22/01/2018 9:22) ?
> > > > >
> > > > >
> > > > > yes. The test ran fine for me
> > > >
> > > > You tried with 0x30000000, it works as well on PPC32.
> > > >
> > > > I'd really like you to try with 0x11000000 which is in the same 
> > > > slice as the 10020000-10030000 range.
> > >
> > > Now that explains is better. But then the requested HEAPBASE was not 
> > > free and hence topdown search got an address in the below range.
> > >
> > > 7efffd000000-7f0000000000 rw-p 00000000 00:0d 1082770 /anon_hugepage 
> > > (deleted)
> > >
> > >
> > > The new range allocated is such that there is no scope for expansion 
> > > of heap if we do a topdown search. But why should that require us to 
> > > change from topdown/bottomup search?
> > >
> > >
> > > 10000000-10010000 r-xp 00000000 fc:00 9044312 /home/kvaneesh/a.out
> > > 10010000-10020000 r--p 00000000 fc:00 9044312 /home/kvaneesh/a.out
> > > 10020000-10030000 rw-p 00010000 fc:00 9044312 /home/kvaneesh/a.out
> > > 7efffd000000-7f0000000000 rw-p 00000000 00:0d 1082770 /anon_hugepage 
> > > (deleted)
> > > 7ffff2d40000-7ffff7d60000 rw-p 00000000 00:00 0
> > > 7ffff7d60000-7ffff7f10000 r-xp 00000000 fc:00 9250090 
> > > /lib/powerpc64le-linux-gnu/libc-2.23.so
> > > 7ffff7f10000-7ffff7f20000 r--p 001a0000 fc:00 9250090 
> > > /lib/powerpc64le-linux-gnu/libc-2.23.so
> > > 7ffff7f20000-7ffff7f30000 rw-p 001b0000 fc:00 9250090 
> > > /lib/powerpc64le-linux-gnu/libc-2.23.so
> > > 7ffff7f40000-7ffff7f60000 r-xp 00000000 fc:00 10754812 
> > > /usr/lib/libhugetlbfs.so.0
> > > 7ffff7f60000-7ffff7f70000 r--p 00010000 fc:00 10754812 
> > > /usr/lib/libhugetlbfs.so.0
> > > 7ffff7f70000-7ffff7f80000 rw-p 00020000 fc:00 10754812 
> > > /usr/lib/libhugetlbfs.so.0
> > > 7ffff7f80000-7ffff7fa0000 r-xp 00000000 00:00 0 [vdso]
> > > 7ffff7fa0000-7ffff7fe0000 r-xp 00000000 fc:00 9250107 
> > > /lib/powerpc64le-linux-gnu/ld-2.23.so
> > > 7ffff7fe0000-7ffff7ff0000 r--p 00030000 fc:00 9250107 
> > > /lib/powerpc64le-linux-gnu/ld-2.23.so
> > > 7ffff7ff0000-7ffff8000000 rw-p 00040000 fc:00 9250107 
> > > /lib/powerpc64le-linux-gnu/ld-2.23.so
> > > 7ffffffd0000-800000000000 rw-p 00000000 00:00 0 [stack]
> > >
> > >
> > > For the specific test, one should pass the HEAPBASE value such that 
> > > it can be expanded if required isn't it ?
> >
> > For the test, yes, it is dumb to pass an unusable HEAPBASE, but what 
> > happens in real life:
> > * PPC32: No HEAPBASE, hugetlbfs defines a HEAPBASE at sbrk(0) + 
> > PAGE_SIZE = 0x10800000 ==> This is in the same slice as already 
> > allocated ==> the kernel does as if mmap() had been called with no 
> > hint address and allocates something unusable instead.
> > * PPC64: No HEAPBASE, hugetlbfs seems to define a HEAPBASE at 
> > 100000000000, which doesn't conflict with an already allocated mapping 
> > ==> it works.
> >
> > Now, when we take the generic case, ie when slice is not activated, 
> > when you call mmap() without a hint address, it allocates a suitable 
> > address because it does bottom-up. Why do differently with slices ?
>
> IIUC that is largely arch dependent, PPC64 always did topdown search. 
> Even for regular non hugetlb mmap it did topdown search. If you set 
> legacy mmap we selected bottom up approach. You can check 
> arch_pick_mmap_layout() for more details. Now x86 is slightly different.
> For the default search if we can't find a mapping address it will try a 
> bottomup search. Having said that if you think libhugetlbfs made 
> assumptions with respect to 8xx and you don't want to break it make
> 8xx unmapped area search bottomup.

Or would there be a way to make libhugetlbfs aware of the slices 
constraints and make it choose a suitable hint address at first try ?

Christophe