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

[Liang, Kan]

On 8/10/2020 6:47 PM, Peter Zijlstra wrote:
> 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.

Yes, the MMU page-size is better here.

I still have some questions regarding MMUPageSize VS. KerelPageSize.
Could you please clarify?

I checked the show_smap code in fs/proc/task_mmu.c. We defined a __weak 
function for vma_mmu_pagesize(), which invokes vma_kernel_pagesize(). 
The comments also say that "In the majority of cases, the page size used 
by the kernel matches the MMU size. On architectures where it differs, 
an architecture-specific 'strong' version of this symbol is required."
I searched the vma_mmu_pagesize(). It seems that PowerPC is the only one 
that defines a 'strong' function. In other words, the MMUPageSize and 
KerelPageSize are the same for X86. However, it seems not true for the 
above compound page cases. Is it a bug for smaps? Or am I missed anything?

Thanks,
Kan