Message-ID: <60ed08a3-d502-4e1a-8e23-33ba0eb4d7bb@arm.com>
Date: Thu, 19 Jun 2025 21:44:41 +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 9:01 pm, Donet Tom wrote:
> eOn Thu, Jun 19, 2025 at 02:32:19PM +0530, Dev Jain wrote:
>> 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?
>>
> Yes. I am doing the same.
>   
>> 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.
>>
> Yes. Got it.
>
> I ran the same test on x86, and what I am seeing is that mmap with a
> hint in this test is always failing and exiting the loop and no high VMA
> is getting created. Ideally mmap should be succeed with hint right?

No, that will succeed only if the CPU has LA57 feature, see arch/x86/include/asm/pgtable_64_types.h,
X86_FEATURE_LA57. On arm64 that will happen only if CPU supports FEAT_LPA2.

So the high address VMAs which you quoted above, were for arm64?

>
>
>> 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.
>
> So, in the test program, is the second mmap (with hint) returning a
> mapped address, or is it failing in your case?
>   
>> 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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