Re: [PATCH] fs/proc/task_mmu: count KSM-placed zero pages in smaps

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

On 7/6/26 14:13, Jinjiang Tu wrote:
> When KSM deduplicates a page to the kernel zero page (i.e. when
> /sys/kernel/mm/ksm/use_zero_pages=1), it places a zero PTE with the dirty
> bit set. vm_normal_page() returns NULL for such PTEs, so smaps_pte_entry()
> bails out before reaching smaps_account() and these pages are never counted
> into mss->ksm. As a result /proc/<pid>/smaps and smaps_rollup report a KSM
> value of 0 even though the region is fully merged.
> 
> Count KSM-placed zero pages directly in smaps_pte_entry() using
> is_ksm_zero_pte().

Hm.

We do expose "ksm_zero_pages" in /proc/<pid>/ksm_stat for calculating the KSM
profit.

Why would we want to expose it here as KSM pages?

An application will notice the difference between "zeropage enabled vs.
disabled" either way because we'll also not account zerpages towards e.g.,
"Anonymous".

So faking that something consumes memory when it actually doesn't consume memory
is wrong?

-- 
Cheers,

David