Re: [PATCH V6 01/16] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

[Peter Zijlstra]

On Mon, Aug 10, 2020 at 06:38:35PM -0400, Liang, Kan wrote:
> On 8/10/2020 5:47 PM, Dave Hansen wrote:

> > It's probably best if we very carefully define up front what is getting
> > reported here.  For instance, I believe we already have some fun cases
> > with huge tmpfs where a compound page is mapped with 4k PTEs.  Kirill
> > also found a few drivers doing this as well.  I think there were also
> > some weird cases for ARM hugetlbfs where there were multiple hardware
> > page table entries mapping a single hugetlbfs page.  These would be
> > cases where compound_head() size would be greater than the size of the
> > leaf paging structure entry.
> > 
> > This is also why we have KerelPageSize and MMUPageSize in /proc/$pid/smaps.
> > 
> > So, is this returning the kernel software page size or the MMU size?
> > 
> 
> This tries to return the kernel software page size. I will add a commit to
> the function. For the above cases, I think they can be detected by
> PageCompound(page). The current code should already cover them. Is my
> understanding correct?

But the rationale for the whole feature was to measure and possibly
drive large page promotion/demotion, which requires the mmu page-size.