Re: [RESEND PATCH] mm: align larger anonymous mappings on THP boundaries

[Ryan Roberts <ryan.roberts@arm.com>]

On 20/01/2024 12:04, Ryan Roberts wrote:
> On 14/12/2023 22:34, Yang Shi wrote:
>> From: Rik van Riel <riel@surriel.com>
>>
>> Align larger anonymous memory mappings on THP boundaries by going through
>> thp_get_unmapped_area if THPs are enabled for the current process.
>>
>> With this patch, larger anonymous mappings are now THP aligned.  When a
>> malloc library allocates a 2MB or larger arena, that arena can now be
>> mapped with THPs right from the start, which can result in better TLB hit
>> rates and execution time.
>>
>> Link: https://lkml.kernel.org/r/20220809142457.4751229f@imladris.surriel.com
>> Signed-off-by: Rik van Riel <riel@surriel.com>
>> Reviewed-by: Yang Shi <shy828301@gmail.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Christopher Lameter <cl@linux.com>
>> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
>> ---
>> This patch was applied to v6.1, but was reverted due to a regression
>> report.  However it turned out the regression was not due to this patch.
>> I ping'ed Andrew to reapply this patch, Andrew may forget it.  This
>> patch helps promote THP, so I rebased it onto the latest mm-unstable.
> 
> Hi Yang,
> 
> I'm not sure what regression you are referring to above, but I'm seeing a
> performance regression in the virtual_address_range mm selftest on arm64, caused
> by this patch (which is now in v6.7).
> 
> I see 2 problems when running the test; 1) it takes much longer to execute, and
> 2) the test fails. Both are related:
> 
> The (first part of the) test allocates as many 1GB anonymous blocks as it can in
> the low 256TB of address space, passing NULL as the addr hint to mmap. Before
> this patch, all allocations were abutted and contained in a single, merged VMA.
> 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.
> 
> I'm not quite sure what the fix is - perhaps __thp_get_unmapped_area() should be
> implemented around vm_unmapped_area(), which can manage the alignment more
> intelligently?
> 
> But until/unless someone comes along with a fix, I think this patch should be
> reverted.

Looks like this patch is also the cause of `ksm_tests -H -s 100` starting to
fail on arm64. I haven't looked in detail, but it passes without the change and
fails with. So this should definitely be reverted, I think.


> 
> Thanks,
> Ryan
> 
> 
>>
>>
>>  mm/mmap.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 9d780f415be3..dd25a2aa94f7 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -2232,6 +2232,9 @@ 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)) {
>> +		/* Ensures that larger anonymous mappings are THP aligned. */
>> +		get_area = thp_get_unmapped_area;
>>  	}
>>  
>>  	addr = get_area(file, addr, len, pgoff, flags);
>