Message-ID: <bad7ec4a-1507-4ec4-996a-ea29d07d47a0@arm.com>
Date: Tue, 23 Jan 2024 09:41:29 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Yang Shi <shy828301@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>,
 Yang Shi <yang@...amperecomputing.com>, riel@...riel.com, cl@...ux.com,
 akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RESEND PATCH] mm: align larger anonymous mappings on THP
 boundaries

On 22/01/2024 19:43, Yang Shi wrote:
> On Mon, Jan 22, 2024 at 3:37 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> On 20/01/2024 16:39, Matthew Wilcox wrote:
>>> On Sat, Jan 20, 2024 at 12:04:27PM +0000, Ryan Roberts wrote:
>>>> However, after this patch, each allocation is in its own VMA, and there is a 2M
>>>> gap between each VMA. This causes 2 problems: 1) mmap becomes MUCH slower
>>>> because there are so many VMAs to check to find a new 1G gap. 2) It fails once
>>>> it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting this limit then
>>>> causes a subsequent calloc() to fail, which causes the test to fail.
>>>>
>>>> Looking at the code, I think the problem is that arm64 selects
>>>> ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area() allocates
>>>> len+2M then always aligns to the bottom of the discovered gap. That causes the
>>>> 2M hole. As far as I can see, x86 allocates bottom up, so you don't get a hole.
>>>
>>> As a quick hack, perhaps
>>> #ifdef ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>>> take-the-top-half
>>> #else
>>> current-take-bottom-half-code
>>> #endif
>>>
>>> ?
> 
> Thanks for the suggestion. It makes sense to me. Doing the alignment
> needs to take into account this.
> 
>>
>> There is a general problem though that there is a trade-off between abutting
>> VMAs, and aligning them to PMD boundaries. This patch has decided that in
>> general the latter is preferable. The case I'm hitting is special though, in
>> that both requirements could be achieved but currently are not.
>>
>> The below fixes it, but I feel like there should be some bitwise magic that
>> would give the correct answer without the conditional - but my head is gone and
>> I can't see it. Any thoughts?
> 
> Thanks Ryan for the patch. TBH I didn't see a bitwise magic without
> the conditional either.
> 
>>
>> Beyond this, though, there is also a latent bug where the offset provided to
>> mmap() is carried all the way through to the get_unmapped_area()
>> impelementation, even for MAP_ANONYMOUS - I'm pretty sure we should be
>> force-zeroing it for MAP_ANONYMOUS? Certainly before this change, for arches
>> that use the default get_unmapped_area(), any non-zero offset would not have
>> been used. But this change starts using it, which is incorrect. That said, there
>> are some arches that override the default get_unmapped_area() and do use the
>> offset. So I'm not sure if this is a bug or a feature that user space can pass
>> an arbitrary value to the implementation for anon memory??
> 
> Thanks for noticing this. If I read the code correctly, the pgoff used
> by some arches to workaround VIPT caches, and it looks like it is for
> shared mapping only (just checked arm and mips). And I believe
> everybody assumes 0 should be used when doing anonymous mapping. The
> offset should have nothing to do with seeking proper unmapped virtual
> area. But the pgoff does make sense for file THP due to the alignment
> requirements. I think it should be zero'ed for anonymous mappings,
> like:
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ff79b1d1564..a9ed353ce627 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1830,6 +1830,7 @@ get_unmapped_area(struct file *file, unsigned
> long addr, unsigned long len,
>                 pgoff = 0;
>                 get_area = shmem_get_unmapped_area;
>         } else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) {
> +               pgoff = 0;
>                 /* Ensures that larger anonymous mappings are THP aligned. */
>                 get_area = thp_get_unmapped_area;
>         }

I think it would be cleaner to just zero pgoff if file==NULL, then it covers the
shared case, the THP case, and the non-THP case properly. I'll prepare a
separate patch for this.


> 
>>
>> Finally, the second test failure I reported (ksm_tests) is actually caused by a
>> bug in the test code, but provoked by this change. So I'll send out a fix for
>> the test code separately.
>>
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 4f542444a91f..68ac54117c77 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -632,7 +632,7 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>>  {
>>         loff_t off_end = off + len;
>>         loff_t off_align = round_up(off, size);
>> -       unsigned long len_pad, ret;
>> +       unsigned long len_pad, ret, off_sub;
>>
>>         if (off_end <= off_align || (off_end - off_align) < size)
>>                 return 0;
>> @@ -658,7 +658,13 @@ static unsigned long __thp_get_unmapped_area(struct file *filp,
>>         if (ret == addr)
>>                 return addr;
>>
>> -       ret += (off - ret) & (size - 1);
>> +       off_sub = (off - ret) & (size - 1);
>> +
>> +       if (current->mm->get_unmapped_area == arch_get_unmapped_area_topdown &&
>> +           !off_sub)
>> +               return ret + size;
>> +
>> +       ret += off_sub;
>>         return ret;
>>  }
> 
> I didn't spot any problem, would you please come up with a formal patch?

Yeah, I'll aim to post today.

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