Re: [PATCH v3 1/2] mm: huge_memory: disallow hugepages if the system-wide THP sysfs settings are disabled

[Baolin Wang]

On 2025/6/24 17:57, Baolin Wang wrote:
> On 2025/6/24 16:41, Dev Jain wrote:
> > On 23/06/25 1:58 pm, Baolin Wang wrote:
> > > When invoking thp_vma_allowable_orders(), the TVA_ENFORCE_SYSFS flag 
> > > is not
> > > specified, we will ignore the THP sysfs settings. Whilst it makes 
> > > sense for the
> > > callers who do not specify this flag, it creates a odd and surprising 
> > > situation
> > > where a sysadmin specifying 'never' for all THP sizes still observing 
> > > THP pages
> > > being allocated and used on the system.
> > >
> > > The motivating case for this is MADV_COLLAPSE. 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.
> > >
> > > 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 its
> > > 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))
> >
> > So the behaviour now is: First check whether THP settings converge to 
> > never.
> > Then, if enforce_sysfs is not set, return immediately. So in this 
> > khugepaged
> > code will it be better to call __thp_vma_allowable_orders()? If the sysfs
> > settings are changed to never before hitting collapse_pte_mapped_thp(),
> > then right now we will return SCAN_VMA_CHECK from here, whereas, the 
> > comment
> > says "regardless of sysfs THP settings", which should include "regardless
> > of whether the sysfs settings say never".
>
> Sounds reasonable to me. Thanks.
>
> I will change thp_vma_allowable_order() to __thp_vma_allowable_orders() 
> in the collapse_pte_mapped_thp() function to maintain consistency with 
> the original logic.
>
> Lorenzo and David, how do you think? Thanks.

After thinking more, since collapse_pte_mapped_thp() is only used for 
file/shmem collapse, changing to __thp_vma_allowable_orders() has no 
effect. So I prefer to leave it as is.