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

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Message-ID: <03c7f5ce-c2ed-7038-3a8b-3bb7a9a4a2dc@c-s.fr>
Date:   Wed, 24 Jan 2018 11:03:07 +0100
From:   Christophe LEROY <christophe.leroy@....fr>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 5/5] powerpc/mm: Fix growth direction for hugepages
 mmaps with slice



Le 24/01/2018 à 10:51, Aneesh Kumar K.V a écrit :
> 
> 
> On 01/24/2018 03:09 PM, Christophe LEROY wrote:
>>
>>
>> Le 24/01/2018 à 10:35, Aneesh Kumar K.V a écrit :
>>>
> 
>>>> 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 ?

Christophe