Re: [PATCH] smaps: Report correct page sizes with THP

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/9/26 21:17, Andi Kleen wrote:
> Recently I wasted quite some time debugging why THP didn't work, when it
> was just smaps always reporting the base page size. It has separate
> counts for (non m) THP, but using them is not always obvious. For
> standard THP the page sizes can be actually derived from the existing
> counts, so do just do that. I left KernelPageSize alone.
> The mixed page size case is reported with a new MMUPageSize2 item.
> This doesn't do anything about mTHP reporting, but even the basic
> smaps is not aware of it so far.
> 
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
>   Documentation/filesystems/proc.rst |  2 +-
>   fs/proc/task_mmu.c                 | 14 +++++++++++++-
>   2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> index 8256e857e2d7..7c776046d15a 100644
> --- a/Documentation/filesystems/proc.rst
> +++ b/Documentation/filesystems/proc.rst
> @@ -483,7 +483,7 @@ entries; the page size used by the MMU when backing a VMA (in most cases,
>   the same as KernelPageSize); the amount of the mapping that is currently
>   resident in RAM (RSS); the process's proportional share of this mapping
>   (PSS); and the number of clean and dirty shared and private pages in the
> -mapping.
> +mapping. If the mapping has multiple page size there might be a MMUPageSize2.
>   
>   The "proportional set size" (PSS) of a process is the count of pages it has
>   in memory, where each page is divided by the number of processes sharing it.
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 26188a4ad1ab..9123e59dcf4c 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1377,7 +1377,19 @@ static int show_smap(struct seq_file *m, void *v)
>   
>   	SEQ_PUT_DEC("Size:           ", vma->vm_end - vma->vm_start);
>   	SEQ_PUT_DEC(" kB\nKernelPageSize: ", vma_kernel_pagesize(vma));
> -	SEQ_PUT_DEC(" kB\nMMUPageSize:    ", vma_mmu_pagesize(vma));
> +
> +	/* Only THP? */
> +	if (mss.shmem_thp + mss.file_thp + mss.anonymous_thp == mss.resident &&
> +	    mss.resident > 0) {
> +		SEQ_PUT_DEC(" kB\nMMUPageSize:    ", HPAGE_PMD_SIZE);
> +	} else {
> +		unsigned ps = vma_mmu_pagesize(vma);
> +		/* Will need adjustments when more THP page sizes are added. */
> +		SEQ_PUT_DEC(" kB\nMMUPageSize:    ", ps);
> +		if (mss.shmem_thp + mss.file_thp + mss.anonymous_thp > 0 &&
> +		    ps != HPAGE_PMD_SIZE)
> +			SEQ_PUT_DEC(" kB\nMMUPageSize2:   ", HPAGE_PMD_SIZE);
> +	}
>   	seq_puts(m, " kB\n");
>   
>   	__show_smap(m, &mss, false);

We have AnonHugePages:, ShmemPmdMapped: and FilePmdMapped: that tell you 
exactly what you want to know.

Especially the mixed thing is just nasty.

Once we go into cont-pte territory (or automatic pte coalescing by 
hardware) it all gets confusing.

Sorry, NAK.

-- 
Cheers,

David