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

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Message-ID: <1f195812-6088-4f20-88f2-d4ab999fae08@lucifer.local>
Date: Mon, 9 Jun 2025 16:12:57 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
        Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com,
        dev.jain@....com, ziy@...dia.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm: huge_memory: disallow hugepages if the
 system-wide THP sysfs settings are disabled

On Mon, Jun 09, 2025 at 02:18:07PM +0800, Baolin Wang wrote:
>
>
> On 2025/6/7 20:21, Lorenzo Stoakes wrote:
> > 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.
>
> Currently, besides MADV_COLLAPSE not setting TVA_ENFORCE_SYSFS, there is
> only one other instance where TVA_ENFORCE_SYSFS is not set, which is in the
> collapse_pte_mapped_thp() function, but I believe this is reasonable from
> the comments:
>
>         /*
>          * If we are here, we've succeeded in replacing all the native pages
>          * in the page cache with a single hugepage. If a mm were to
> fault-in
>          * this memory (mapped by a suitably aligned VMA), we'd get the
> hugepage
>          * and map it by a PMD, regardless of sysfs THP settings. As such,
> let's
>          * analogously elide sysfs THP settings here.
>          */
>         if (!thp_vma_allowable_order(vma, vma->vm_flags, 0, PMD_ORDER))
>                 return SCAN_VMA_CHECK;

Thanks for referencing that - it's a nicely worded comment that does explain it
indeed :)

Definitely worth mentioning that in the commit msg I think.

>
> >
> > 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.
>
> Sure.

Thanks!

>
> >
> > (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.
>
> Sure, will do. Thanks for reviewing.

Thanks!