Re: [PATCH 2/3] fs/proc/task_mmu: don't read young/dirty from a non-present PMD in smaps

[Zi Yan]

On 11 Nov 2026, at 6:38, Usama Arif wrote:

> smaps_pmd_entry() passes pmd_young(*pmd) and pmd_dirty(*pmd) to
> smaps_account() for every PMD, including migration and device-private
> entries, where those bits hold part of the swap offset rather than
> accessed/dirty state. smaps_pte_entry() only reads them when the entry
> is present.
>
> On x86_64 the offset is stored inverted across bits 9-58, which overlap
> _PAGE_SAVED_DIRTY, so pmd_dirty() returns garbage: a clean 2MB
> device-private mapping is accounted as 2048 kB dirty, and lazyfree
> accounting is suppressed. pgtable_64.h also warns that the hardware can
> set the accessed and dirty bits on non-present entries by mistake.
>
> Only read the bits when the PMD is present.
>
> Fixes: c94b6923fa0a ("/proc/PID/smaps: Add PMD migration entry parsing")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>
> ---
>  fs/proc/task_mmu.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
Makes sense.

Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>

Best Regards,
Yan, Zi