Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

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

A couple follow up points that occurred to me:

On Sat, Jun 07, 2025 at 12:55:19PM +0100, Lorenzo Stoakes wrote:
> Not related to your patch at all, but man this whole thing (thp allowed orders)
> needs significant improvement, it seems always perversely complicated for a
> relatively simple operation.
>
> Overall I LOVE what you're doing here, but I feel we can clarify things a
> little while we're at it to make it clear exactly what we're doing.
>
> This is a very important change so forgive my fiddling about here but I'm
> hoping we can take the opportunity to make things a little simpler!
>
> On Thu, Jun 05, 2025 at 04:00:58PM +0800, Baolin Wang wrote:
> > The MADV_COLLAPSE will ignore the system-wide Anon THP sysfs settings, which
> > means that even though we have disabled the Anon THP configuration, MADV_COLLAPSE
> > will still attempt to collapse into a Anon THP. This violates the rule we have
> > agreed upon: never means never.
> >
> > Another rule for madvise, referring to David's suggestion: “allowing for collapsing
> > in a VM without VM_HUGEPAGE in the "madvise" mode would be fine".
>
> I'm generally not sure it's worth talking only about MADV_COLLAPSE here when
> you're changing what THP is permitted across the board, I may have missed some
> discussion and forgive me if so, but what is special about MADV_COLLAPSE's use
> of thp_vma_allowable_orders() that makes it ignore 'never's moreso than other
> users?

I'd mention that MADV_COLLAPSE is special because of not specifying
TVA_ENFORCE_SYSFS but you are making this change across the board for all
callers who do not specify this.

I'd also CLEARLY mention that you handle David's request re: madvise by
restricting yourself to checking only for NEVER and retaining the existing logic
of not enforcing sysfs settings when TVA_ENFORCE_SYSFS, which includes not
checking the VMA for VM_HUGEPAGE if the madvise mode is enabled.

(i.e. addressing David's request).

[snip]

> I feel this is compressing a lot of logic in a way that took me several
> readings to understand (hey I might not be the smartest cookie in the jar,
> but we need to account for all levels of kernel developer ;)
>
> I feel like we can make things a lot clearer here by separating out with a
> helper function (means we can drop some indentation too), and also take
> advantage of the fact that, if orders == 0, __thp_vma_allowable_orders()
> exits with 0 early so no need for us to do so ourselves:
>
> /* 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)
> {
> 	unsigned long always = READ_ONCE(huge_anon_orders_always);
> 	unsigned long madvise = READ_ONCE(huge_anon_orders_madvise);
> 	unsigned long inherit = READ_ONCE(huge_anon_orders_inherit);;
> 	bool inherit_enabled = hugepage_global_enabled();
> 	bool has_madvise =  vm_flags & VM_HUGEPAGE;
> 	unsigned long mask = always | madvise;
>
> 	mask = always | madvise;
> 	if (inherit_enabled)
> 		mask |= inherit;
>
> 	/* All set to/inherit NEVER - never means never globally, abort. */
> 	if (!(mask & orders))
> 		return 0;
>
> 	/* Otherwise, we only enforce sysfs settings if asked. */

Perhaps worth adding a comment here noting that, 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;
>
> 	mask = always;
> 	if (has_madvise)
> 		mask |= madvise;
> 	if (hugepage_global_always() || (has_madvise && inherit_enabled))
> 		mask |= inherit;
>
> 	return orders & mask;
> }
>
> ...
>
> static inline
> unsigned long thp_vma_allowable_orders(struct vm_area_struct *vma,
> 				       unsigned long vm_flags,
> 				       unsigned long tva_flags,
> 				       unsigned long orders)
> {
> 	if (vma_is_anonymous(vma))
> 		orders = __thp_mask_anon_orders(vm_flags, tva_flags, orders);
>
> 	return __thp_vma_allowable_orders(vma, vm_flags, tva_flags, orders);
> }
>
> >
> > --
> > 2.43.5
> >