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

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

OK overall the logic looks good now I realise the mistake I made is that
the thp_vma_allowable_orders() check is for the vma_is_anonymous() case
only.

On Mon, Jun 09, 2025 at 02:31:46PM +0800, Baolin Wang wrote:
>
>
> On 2025/6/7 20:14, Lorenzo Stoakes wrote:
> > On Thu, Jun 05, 2025 at 04:00:59PM +0800, Baolin Wang wrote:
> > > The MADV_COLLAPSE will ignore the system-wide shmem THP sysfs settings, which
> > > means that even though we have disabled the shmem THP configuration, MADV_COLLAPSE
> > > will still attempt to collapse into a shmem THP. This violates the rule we have
> > > agreed upon: never means never.
> >
> > Ugh that we have separate shmem logic. And split between huge_memory.c and
> > shmem.c too :)) Again, not your fault, just a general moan about existing
> > stuff :P
> >
> > >
> > > 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".
> >
> > Hm I'm not sure if this is enforced is it? I may have missed something here
> > however.
> >
> > >
> > > Then the current strategy is:
> > >
> > > For shmem, if none of always, madvise, within_size, and inherit have enabled
> > > PMD-sized THP, then MADV_COLLAPSE will be prohibited from collapsing PMD-sized THP.
> >
> > Again, is this just MADV_COLLAPSE? Surely this is a general change?
> >
> > We should be clear that we are not explicitly limiting ourselves to
> > MADV_COLLAPSE here.
> >
> > You shoudl clearly indicate that the MADV_COLLAPSE case DOESN'T set
> > TVA_ENFORCE_SYSFS and that's the key difference here.
>
> Sure.
>

Thanks!

> > > For tmpfs, if the mount option is set with the 'huge=never' parameter, then
> > > MADV_COLLAPSE will be prohibited from collapsing PMD-sized THP.
> > >
> > > Acked-by: Zi Yan <ziy@nvidia.com>
> > > Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > > ---
> > >   mm/huge_memory.c | 2 +-
> > >   mm/shmem.c       | 6 +++---
> > >   2 files changed, 4 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index d3e66136e41a..a8cfa37cae72 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -166,7 +166,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> > >   	 * own flags.
> > >   	 */
> > >   	if (!in_pf && shmem_file(vma->vm_file))
> > > -		return shmem_allowable_huge_orders(file_inode(vma->vm_file),
> > > +		return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file),
> > >   						   vma, vma->vm_pgoff, 0,
> > >   						   !enforce_sysfs);
> >
> > Did you mean to do &&?
>
> No. It might be worth having a separate fix patch here, because the original
> logic is incorrect and needs to perform an '&' operation with ’orders‘.
>
> This change should be a general change.

Ah yeah, I did think that _perhaps_ it could be. I think it would make
sense to separate out into another patch albeit very small, just so we can
separate your further changes from the fix for this.

>
> > Also, is this achieving what you want to achieve? Is it necessary? The
> > changes in patch 1/2 enforce the global settings and before this code in
> > __thp_vma_allowable_orders():
> >
> > unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
> > 					 unsigned long vm_flags,
> > 					 unsigned long tva_flags,
> > 					 unsigned long orders)
> > {
> > 	... (no early exits) ...
> >
> > 	orders &= supported_orders;
> > 	if (!orders)
> > 		return 0;
> >
> > 	...
> > }
> >
> > So if orders == 0 due to the changes in thp_vma_allowable_orders(), which
> > is the only caller of __thp_vma_allowable_orders() then we _always_ exit
> > early here before we get to this shmem_allowable_huge_orders() code.
>
> Not for this case. Since shmem already supports mTHP, this is to check
> whether the 'orders' are enabled in the shmem mTHP configuration. For
> example, shmem mTHP might only enable 64K mTHP, which obviously does not
> allow PMD-sized THP to collapse.

Yeah sorry I get it now thp_vma_allowable_orders() does a
vma_is_anonymous() predicate. Doh! :P

God what a mess this is (not your fault, pre-existing obviously :P)

Yeah le sigh.

>
> > > diff --git a/mm/shmem.c b/mm/shmem.c
> > > index 4b42419ce6b2..9af45d4e27e6 100644
> > > --- a/mm/shmem.c
> > > +++ b/mm/shmem.c
> > > @@ -625,7 +625,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
> > >   		return 0;
> > >   	if (shmem_huge == SHMEM_HUGE_DENY)
> > >   		return 0;
> > > -	if (shmem_huge_force || shmem_huge == SHMEM_HUGE_FORCE)
> > > +	if (shmem_huge == SHMEM_HUGE_FORCE)
> > >   		return maybe_pmd_order;

OK I get it now, this means the !check sysfs doesn't just get
actioned... :)

> > >
> > >   	/*
> > > @@ -660,7 +660,7 @@ static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index
> > >
> > >   		fallthrough;
> > >   	case SHMEM_HUGE_ADVISE:
> > > -		if (vm_flags & VM_HUGEPAGE)
> > > +		if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
> > >   			return maybe_pmd_order;
> > >   		fallthrough;
> > >   	default:
> > > @@ -1790,7 +1790,7 @@ unsigned long shmem_allowable_huge_orders(struct inode *inode,
> > >   	/* Allow mTHP that will be fully within i_size. */
> > >   	mask |= shmem_get_orders_within_size(inode, within_size_orders, index, 0);
> > >
> > > -	if (vm_flags & VM_HUGEPAGE)
> > > +	if (shmem_huge_force || (vm_flags & VM_HUGEPAGE))
> > >   		mask |= READ_ONCE(huge_shmem_orders_madvise);
> >
> > I'm also not sure these are necessary:
> >
> > The only path that can set shmem_huge_force is __thp_vma_allowable_orders()
> > -> shmem_allowable_huge_orders() -> shmem_huge_global_enabled() and then
> > only if !(tva_flags & TVA_ENFORCE_SYSFS) and as stated above we already
> > cover off this case by early exiting __thp_vma_allowable_orders() if orders
> > == 0 as established in patch 1/2.
>
> Not ture. Shmem has its own separate mTHP sysfs setting, which is different
> from the mTHP sysfs setting for anonymous pages mentioned earlier. These
> checks are in the shmem file. You can check more for shmem mTHP in
> Documentation/admin-guide/mm/transhuge.rst :)

Ah yeah the issue is if (vma_is_anonymous())... doh!

The stack trace is correct though, this is the only place we do it:

~~

static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index,
					      loff_t write_end, bool shmem_huge_force,
					      struct vm_area_struct *vma,
					      unsigned long vm_flags)

Is called from shmem_getattr():

	if (shmem_huge_global_enabled(inode, 0, 0, false, NULL, 0))
		stat->blksize = HPAGE_PMD_SIZE;

Note that smem_huge_force == false here

And shmem_allowable_huge_orders():

unsigned long shmem_allowable_huge_orders(struct inode *inode,
				struct vm_area_struct *vma, pgoff_t index,
				loff_t write_end, bool shmem_huge_force)

	global_orders = shmem_huge_global_enabled(inode, index, write_end,
						  shmem_huge_force, vma, vm_flags);

Which forwards 'shmem_huge_force'.

In shmem_get_folow_gfp():

	orders = shmem_allowable_huge_orders(inode, vma, index, write_end, false);

Note that shmem_huge_force == false.

In __thp_vma_allowable_orders();

unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
					 unsigned long vm_flags,
					 unsigned long tva_flags,
					 unsigned long orders)
{
	...
	bool enforce_sysfs = tva_flags & TVA_ENFORCE_SYSFS;

	...

	if (!in_pf && shmem_file(vma->vm_file))
		return shmem_allowable_huge_orders(file_inode(vma->vm_file),
						   vma, vma->vm_pgoff, 0,
						   !enforce_sysfs);

So we set shmem_huge_force only if TVA_ENFORCE_SYSFS is not set in tva_flags passed to __thp_vma_allowable_orders()

The only caller of __thp_vma_allowable_orders() is thp_vma_allowable_orders().

~~

But yeah we do need to do shmem things... sorry my mistake.