Re: [PATCH v3 33/40] mm/madvise: use predicates for madvise(..., MADV_DOFORK)

[Zi Yan]

On Thu Sep 17, 2026 at 12:22 PM EDT, Lorenzo Stoakes (ARM) wrote:
> Make it clear what we're blocking in MADV_DOFORK. Previously we simply
> disallowed VM_SPECIAL i.e. kernel-owned mappings, fixed mappings and
> VMA_IO_BIT.
>
> Now the invariant is established that only kernel-owned mappings can set
> VMA_IO_BIT, the VMA_IO_BIT check is redundant.
>
> The rest is equivalent to testing for a kernel-owned or fixed mapping,
> i.e. exactly the same check as whether the VMA is permitted to be merged.
>
> This was established by commit 0b2758f48f22 ("Require (reasonably) normal
> mappings for MADV_DOFORK") containing my hands-down favourite call out of
> all time.
>
> Express the same thing differently - if we wouldn't be allowed to merge it,
> then we aren't allowed to manipulate CoW behaviour on fork.
>
> No functional change intended.
>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---
>  mm/madvise.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

Makes sense.

Acked-by: Zi Yan <ziy@xxxxxxxxxx>

-- 
Best Regards,
Yan, Zi