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

[Dev Jain]

On 24/06/25 7:38 pm, Baolin Wang wrote:
> 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.


Oops my bad, thanks.