Re: [PATCH v2 0/2] fix MADV_COLLAPSE issue if THP settings are disabled

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, Jun 13, 2025 at 03:23:19PM +0100, Usama Arif wrote:
>
>
> On 05/06/2025 09:00, Baolin Wang wrote:
> > As we discussed in the previous thread [1], the MADV_COLLAPSE will ignore
> > the system-wide anon/shmem THP sysfs settings, which means that even though
> > we have disabled the anon/shmem THP configuration, MADV_COLLAPSE will still
> > attempt to collapse into a anon/shmem THP. This violates the rule we have
> > agreed upon: never means never. This patch set will address this issue.
>
> Hi Baolin,
>
> I know never means never, but I also thought that the per-size toggles had
> priority over the system ones. This was discussed in [1] as well.
>
> My understanding with these patches is that if we have:
>
> [root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/enabled
> always madvise [never]
> [root@vm4 vmuser]# cat /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> always inherit [madvise] never
>
> Than without these patches we get a hugepage when we do MADV_HUGEPAGE, but with
> these we won't get a hugepage anymore eventhough hugepages-2048kB/enabled is set
> to madvise?

This isn't correct, madvise at a specific pagesize will still be permitted for
MADV_COLLAPSE.

In current contender for this patch:

/* Strictly mask requested anonymous orders according to sysfs settings. */
static inline unsigned long __thp_mask_anon_orders(unsigned long vm_flags,
                unsigned long tva_flags, unsigned long orders)
{
        const unsigned long always = READ_ONCE(huge_anon_orders_always);
        const unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
        const unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);;
        const unsigned long never = ~(always | madvise | inherit);

Note that madvise is considered here.

        const bool inherit_never = !hugepage_global_enabled();

        /* Disallow orders that are set to NEVER directly ... */
        orders &= ~never;

        /* ... or through inheritance (global == NEVER). */
        if (inherit_never)
                orders &= ~inherit;

        /*
         * Otherwise, we only enforce sysfs settings if asked. In addition,
         * if the user sets a sysfs mode of madvise and if TVA_ENFORCE_SYSFS
         * is not set, we don't bother checking whether the VMA has VM_HUGEPAGE
         * set.
         */
        if (!(tva_flags & TVA_ENFORCE_SYSFS))
                return orders;

And then if !TVA_ENFORCE_SYSFS (e.g. MADV_COLLAPSE case), an madvise should
succeed fine.

>
> I know this isn't ABI, but this would break existing expectations.
> (For e.g. we have certain 64K page size arm machines with global enabled = never and
> 2M = madvise, and we want 2M hugepages to fault at madvise).
> If the whole thing was being implemented from scratch, we should have definitely
> done it this way, but this can give a people a nasty surprise when they upgrade
> the kernel and suddenly stop getting hugepages.
>
> [1] https://lore.kernel.org/all/97702ff0-fc50-4779-bfa8-83dc42352db1@redhat.com/
>
> Thanks,
> Usama