Re: [PATCH v5 14/14] mm: thp: always enable mTHP support

[Lance Yang]

On 2026/6/3 15:23, Baolin Wang wrote:
> On 6/3/26 2:47 PM, Lance Yang wrote:
> > Hi Luiz,
> >
> > SHMEM_HUGE_FORCE still assumes PMD order in a few places.
> >
> > Is that expected?
> >
> > shmem_init() only sets the default inherit mask when PMD leaves are
> > available.
> >
> >     if (!shmem_orders_configured && pgtable_has_pmd_leaves())
> >         huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
> >
> > But shmem_parse_huge() rejects "force" unless the mask is exactly PMD
> > order.
> >
> >     if (huge == SHMEM_HUGE_FORCE &&
> >         huge_shmem_orders_inherit != BIT(HPAGE_PMD_ORDER))
> >         return -EINVAL;
> >
> > Even if "force" is selected, shmem_huge_global_enabled() still return
> > only PMD order.
> >
> >     if (shmem_huge_force || shmem_huge == SHMEM_HUGE_FORCE)
> >         return maybe_pmd_order;
> >
> > and shmem_allowable_huge_orders() mask it out.
> >
> >     if (!pgtable_has_pmd_leaves())
> >         disabled_orders = BIT(PMD_ORDER);
> >
> >     if (!vma || !vma_is_anon_shmem(vma))
> >         return global_orders & ~disabled_orders;
> >
> > For anon shmem, it can also return 0 for same reason.
> >
> >     if (shmem_huge == SHMEM_HUGE_FORCE)
> >         return READ_ONCE(huge_shmem_orders_inherit) & ~disabled_orders;
> >
> > Should SHMEM_HUGE_FORCE use the available mTHP orders below PMD when
> > pgtable_has_pmd_leaves() is false?
>
> No. That's exactly the strategy we agreed on after lots of discussion:
> 1. The options 'force' and 'deny' are dropped for mTHP interfaces, which 
> are rather testing artifacts from the old ages.
>
> 2. If top level value is 'force', then only PMD-sized hugepages have 
> enabled="inherit", otherwise configuration will be failed and vice versa.
>
> You can check commit 4b98995530b7 ("mm: shmem: add multi-size THP sysfs 
> interface for anonymous shmem") for more information.

Ah, thanks Baolin, good to know that :D

Cheers, Lance