Re: [PATCH] mm: map maximum pages possible in finish_fault

[Dev Jain <dev.jain@arm.com>]

On 06/02/26 8:52 pm, Matthew Wilcox wrote:
> On Fri, Feb 06, 2026 at 07:26:48PM +0530, Dev Jain wrote:
>> We test the patch with the following userspace program. A shmem VMA of
>> 2M is created, and faulted in, with sysfs setting
>> hugepages-2048k/shmem_enabled = always, so that the pagecache is populated
>> with a 2M folio. Then, a 64K VMA is created, and we fault on each page.
>> Then, we do MADV_DONTNEED to zap the pagetable, so that we can fault again
>> in the next iteration. We measure the accumulated time taken during
>> faulting the VMA.
>>
>> On arm64,
>>
>> without patch:
>> Total time taken by inner loop: 4701721766 ns
>>
>> with patch:
>> Total time taken by inner loop: 516043507 ns
>>
>> giving a 9x improvement.
> It's nice that you can construct a test-case that shows improvement, but
> is there any real workload that benefits from this?

I can try to measure this. But, I constructed that testcase to test the
code path, not to show a perf boost (although the boost is obvious enough
so why not show it). As I say in the description:

"Align finish_fault with filemap_map_pages, and map as many pages as
possible, without crossing VMA/PMD/file boundaries."

The patch should rather be seen as an extension to 19773df031bc
("mm/fault: try to map the entire file folio in finish_fault()").
The code which my patch removes, was added when the norm was to still
perform per-page fault, the argument being, RSS inflation.

Perhaps I can polish the patch description so that it clearly mentions
what the objective is.