Message-ID: <af2618dc-b778-45e6-867e-9ba3a79398c7@arm.com>
Date: Thu, 19 Jun 2025 14:32:19 +0530
From: Dev Jain <dev.jain@....com>
To: Donet Tom <donettom@...ux.ibm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Aboorva Devarajan <aboorvad@...ux.ibm.com>, akpm@...ux-foundation.org,
 Liam.Howlett@...cle.com, shuah@...nel.org, pfalcato@...e.de,
 david@...hat.com, ziy@...dia.com, baolin.wang@...ux.alibaba.com,
 npache@...hat.com, ryan.roberts@....com, baohua@...nel.org,
 linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
 linux-kernel@...r.kernel.org, ritesh.list@...il.com
Subject: Re: [PATCH 1/6] mm/selftests: Fix virtual_address_range test issues.


On 19/06/25 1:53 pm, Donet Tom wrote:
> On Wed, Jun 18, 2025 at 08:13:54PM +0530, Dev Jain wrote:
>> On 18/06/25 8:05 pm, Lorenzo Stoakes wrote:
>>> On Wed, Jun 18, 2025 at 07:47:18PM +0530, Dev Jain wrote:
>>>> On 18/06/25 7:37 pm, Lorenzo Stoakes wrote:
>>>>> On Wed, Jun 18, 2025 at 07:28:16PM +0530, Dev Jain wrote:
>>>>>> On 18/06/25 5:27 pm, Lorenzo Stoakes wrote:
>>>>>>> On Wed, Jun 18, 2025 at 05:15:50PM +0530, Dev Jain wrote:
>>>>>>> Are you accounting for sys.max_map_count? If not, then you'll be hitting that
>>>>>>> first.
>>>>>> run_vmtests.sh will run the test in overcommit mode so that won't be an issue.
>>>>> Umm, what? You mean overcommit all mode, and that has no bearing on the max
>>>>> mapping count check.
>>>>>
>>>>> In do_mmap():
>>>>>
>>>>> 	/* Too many mappings? */
>>>>> 	if (mm->map_count > sysctl_max_map_count)
>>>>> 		return -ENOMEM;
>>>>>
>>>>>
>>>>> As well as numerous other checks in mm/vma.c.
>>>> Ah sorry, didn't look at the code properly just assumed that overcommit_always meant overriding
>>>> this.
>>> No problem! It's hard to be aware of everything in mm :)
>>>
>>>>> I'm not sure why an overcommit toggle is even necessary when you could use
>>>>> MAP_NORESERVE or simply map PROT_NONE to avoid the OVERCOMMIT_GUESS limits?
>>>>>
>>>>> I'm pretty confused as to what this test is really achieving honestly. This
>>>>> isn't a useful way of asserting mmap() behaviour as far as I can tell.
>>>> Well, seems like a useful way to me at least : ) Not sure if you are in the mood
>>>> to discuss that but if you'd like me to explain from start to end what the test
>>>> is doing, I can do that : )
>>>>
>>> I just don't have time right now, I guess I'll have to come back to it
>>> later... it's not the end of the world for it to be iffy in my view as long as
>>> it passes, but it might just not be of great value.
>>>
>>> Philosophically I'd rather we didn't assert internal implementation details like
>>> where we place mappings in userland memory. At no point do we promise to not
>>> leave larger gaps if we feel like it :)
>> You have a fair point. Anyhow a debate for another day.
>>
>>> I'm guessing, reading more, the _real_ test here is some mathematical assertion
>>> about layout from HIGH_ADDR_SHIFT -> end of address space when using hints.
>>>
>>> But again I'm not sure that achieves much and again also is asserting internal
>>> implementation details.
>>>
>>> Correct behaviour of this kind of thing probably better belongs to tests in the
>>> userland VMA testing I'd say.
>>>
>>> Sorry I don't mean to do down work you've done before, just giving an honest
>>> technical appraisal!
>> Nah, it will be rather hilarious to see it all go down the drain xD
>>
>>> Anyway don't let this block work to fix the test if it's failing. We can revisit
>>> this later.
>> Sure. @Aboorva and Donet, I still believe that the correct approach is to elide
>> the gap check at the crossing boundary. What do you think?
>>
> One problem I am seeing with this approach is that, since the hint address
> is generated randomly, the VMAs are also being created at randomly based on
> the hint address.So, for the VMAs created at high addresses, we cannot guarantee
> that the gaps between them will be aligned to MAP_CHUNK_SIZE.
>
> High address VMAs
> -----------------
> 1000000000000-1000040000000 r--p 00000000 00:00 0
> 2000000000000-2000040000000 r--p 00000000 00:00 0
> 4000000000000-4000040000000 r--p 00000000 00:00 0
> 8000000000000-8000040000000 r--p 00000000 00:00 0
> e80009d260000-fffff9d260000 r--p 00000000 00:00 0

Just confirming, the correct way to test this will be, put a sleep
after the VA gets exhausted by the test, and then examine /proc/pid/maps -
are you doing something similar?

The random generation of the hint addr should not be a problem - if we
cannot satisfy the request at addr, then the algorithm falls back to
the original approach.

FYI in arch/x86/kernel/sys_x86_64.c :

    * If hint address is above DEFAULT_MAP_WINDOW, look for unmapped area
          * in the full address space.

Same happens for arm64; if we give a high addr hint, and the high VA space
has been exhausted, then we look for free space in the low VA space.

The only thing I am not sure about is the border. I remember witnessing weird
behaviour when I used to test with 16K page config on arm64, across the
border.
  

>
> I have a different approach to solve this issue.
>
>  From 0 to 128TB, we map memory directly without using any hint. For the range above
> 256TB up to 512TB, we perform the mapping using hint addresses. In the current test,
> we use random hint addresses, but I have modified it to generate hint addresses linearly
> starting from 128TB.
>
> With this change:
>
> The 0–128TB range is mapped without hints and verified accordingly.
>
> The 128TB–512TB range is mapped using linear hint addresses and then verified.
>
> Below are the VMAs obtained with this approach:
>
> 10000000-10010000 r-xp 00000000 fd:05 135019531
> 10010000-10020000 r--p 00000000 fd:05 135019531
> 10020000-10030000 rw-p 00010000 fd:05 135019531
> 20000000-10020000000 r--p 00000000 00:00 0
> 10020800000-10020830000 rw-p 00000000 00:00 0
> 1004bcf0000-1004c000000 rw-p 00000000 00:00 0
> 1004c000000-7fff8c000000 r--p 00000000 00:00 0
> 7fff8c130000-7fff8c360000 r-xp 00000000 fd:00 792355
> 7fff8c360000-7fff8c370000 r--p 00230000 fd:00 792355
> 7fff8c370000-7fff8c380000 rw-p 00240000 fd:00 792355
> 7fff8c380000-7fff8c460000 r-xp 00000000 fd:00 792358
> 7fff8c460000-7fff8c470000 r--p 000d0000 fd:00 792358
> 7fff8c470000-7fff8c480000 rw-p 000e0000 fd:00 792358
> 7fff8c490000-7fff8c4d0000 r--p 00000000 00:00 0
> 7fff8c4d0000-7fff8c4e0000 r-xp 00000000 00:00 0
> 7fff8c4e0000-7fff8c530000 r-xp 00000000 fd:00 792351
> 7fff8c530000-7fff8c540000 r--p 00040000 fd:00 792351
> 7fff8c540000-7fff8c550000 rw-p 00050000 fd:00 792351
> 7fff8d000000-7fffcd000000 r--p 00000000 00:00 0
> 7fffe9c80000-7fffe9d90000 rw-p 00000000 00:00 0
> 800000000000-2000000000000 r--p 00000000 00:00 0    -> High Address (128TB to 512TB)
>
> diff --git a/tools/testing/selftests/mm/virtual_address_range.c b/tools/testing/selftests/mm/virtual_address_range.c
> index 4c4c35eac15e..0be008cba4b0 100644
> --- a/tools/testing/selftests/mm/virtual_address_range.c
> +++ b/tools/testing/selftests/mm/virtual_address_range.c
> @@ -56,21 +56,21 @@
>   
>   #ifdef __aarch64__
>   #define HIGH_ADDR_MARK  ADDR_MARK_256TB
> -#define HIGH_ADDR_SHIFT 49
> +#define HIGH_ADDR_SHIFT 48
>   #define NR_CHUNKS_LOW   NR_CHUNKS_256TB
>   #define NR_CHUNKS_HIGH  NR_CHUNKS_3840TB
>   #else
>   #define HIGH_ADDR_MARK  ADDR_MARK_128TB
> -#define HIGH_ADDR_SHIFT 48
> +#define HIGH_ADDR_SHIFT 47
>   #define NR_CHUNKS_LOW   NR_CHUNKS_128TB
>   #define NR_CHUNKS_HIGH  NR_CHUNKS_384TB
>   #endif
>   
> -static char *hint_addr(void)
> +static char *hint_addr(int hint)
>   {
> -       int bits = HIGH_ADDR_SHIFT + rand() % (63 - HIGH_ADDR_SHIFT);
> +       unsigned long addr = ((1UL << HIGH_ADDR_SHIFT) + (hint * MAP_CHUNK_SIZE));
>   
> -       return (char *) (1UL << bits);
> +       return (char *) (addr);
>   }
>   
>   static void validate_addr(char *ptr, int high_addr)
> @@ -217,7 +217,7 @@ int main(int argc, char *argv[])
>          }
>   
>          for (i = 0; i < NR_CHUNKS_HIGH; i++) {
> -               hint = hint_addr();
> +               hint = hint_addr(i);
>                  hptr[i] = mmap(hint, MAP_CHUNK_SIZE, PROT_READ,
>                                 MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>
>
>
> Can we fix it this way?
>   

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