Re: [RESEND v7 15/29] fs/proc: account PMD swap entries in smaps

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

On 9/14/26 14:28, Usama Arif wrote:
> smaps_pmd_entry() derives a page from a non-present PMD and gives up if it
> cannot. A PMD swap entry carries no PFN, so a swapped-out THP is accounted
> nowhere: its bytes show up in neither Swap nor SwapPss, and VmSwap silently
> loses them.
> 
> The slots of a PMD swap entry can have different swap reference counts, so
> SwapPss has to be computed one slot at a time rather than divided once.
> Factor that loop out of smaps_pte_entry() and call it from both levels,
> then return early: a swapped-out THP is not resident and must not be added
> to AnonHugePages or Rss.
> 
> pagemap needs no equivalent change - it already emits PM_SWAP and
> type | offset+idx for a PFN-less softleaf entry - but its
> thp_migration_supported() gate answers a different question than the decode
> requires, so use pmd_is_valid_softleaf() there instead.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
>  fs/proc/task_mmu.c | 42 ++++++++++++++++++++++++++++--------------
>  1 file changed, 28 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 24425e2308951..aa3f4d54a8580 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -957,6 +957,27 @@ static void smaps_pte_hole_lookup(unsigned long addr, struct mm_walk *walk)
>  #endif
>  }
>  
> +static void smaps_account_swap(struct mem_size_stats *mss,
> +			       softleaf_t entry, unsigned long size)

Wouldn't consuming nr_pages be more consistent with other functions that consume
swap entry ranges?

> +{
> +	unsigned long nr_pages = size >> PAGE_SHIFT;
> +
> +	mss->swap += size;
> +	do {
> +		int mapcount = swp_swapcount(entry);

While at it, can you please call that swapcount?

> +
> +		if (mapcount >= 2) {
> +			u64 pss_delta = (u64)PAGE_SIZE << PSS_SHIFT;
> +
> +			do_div(pss_delta, mapcount);
> +			mss->swap_pss += pss_delta;
> +		} else {
> +			mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
> +		}
> +		entry.val++;
> +	} while (--nr_pages);
> +}


[...]

> @@ -2000,7 +2014,7 @@ static int pagemap_pmd_range_thp(pmd_t *pmdp, unsigned long addr,
>  			flags |= PM_UFFD_WP;
>  		if (pm->show_pfn)
>  			frame = pmd_pfn(pmd) + idx;
> -	} else if (thp_migration_supported()) {
> +	} else if (pmd_is_valid_softleaf(pmd)) {
>  		const softleaf_t entry = softleaf_from_pmd(pmd);
>  		unsigned long offset;
>  

Interesting, the function already seems to do what we want. Weird that we
indicate PM_SWAP for migration entries ...

Also, are device_private PMD's handled correctly? *doubt*

[maybe there were patches, not sure]

-- 
Cheers,

David