Re: [PATCH v1 02/10] mm/khugepaged: remove READ_ONLY_THP_FOR_FS check

Lorenzo Stoakes (Oracle)
2026-03-27T14:31:13Z

On Fri, Mar 27, 2026 at 10:26:53PM +0800, Baolin Wang wrote:
>
>
> On 3/27/26 10:12 PM, Lorenzo Stoakes (Oracle) wrote:
> > On Fri, Mar 27, 2026 at 09:45:03PM +0800, Baolin Wang wrote:
> > >
> > >
> > > On 3/27/26 8:02 PM, Lorenzo Stoakes (Oracle) wrote:
> > > > On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote:
> > > > >
> > > > >
> > > > > On 3/27/26 9:42 AM, Zi Yan wrote:
> > > > > > collapse_file() requires FSes supporting large folio with at least
> > > > > > PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem with
> > > > > > huge option turned on also sets large folio order on mapping, so the check
> > > > > > also applies to shmem.
> > > > > >
> > > > > > While at it, replace VM_BUG_ON with returning failure values.
> > > > > >
> > > > > > Signed-off-by: Zi Yan <ziy@nvidia.com>
> > > > > > ---
> > > > > > mm/khugepaged.c | 7 +++++--
> > > > > > 1 file changed, 5 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > > > > index d06d84219e1b..45b12ffb1550 100644
> > > > > > --- a/mm/khugepaged.c
> > > > > > +++ b/mm/khugepaged.c
> > > > > > @@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct mm_struct *mm, unsigned long addr,
> > > > > > int nr_none = 0;
> > > > > > bool is_shmem = shmem_file(file);
> > > > > > - VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && !is_shmem);
> > > > > > - VM_BUG_ON(start & (HPAGE_PMD_NR - 1));
> > > > > > + /* "huge" shmem sets mapping folio order and passes the check below */
> > > > > > + if (mapping_max_folio_order(mapping) < PMD_ORDER)
> > > > > > + return SCAN_FAIL;
> > > > >
> > > > > This is not true for anonymous shmem, since its large order allocation logic
> > > > > is similar to anonymous memory. That means it will not call
> > > > > mapping_set_large_folios() for anonymous shmem.
> > > > >
> > > > > So I think the check should be:
> > > > >
> > > > > if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
> > > > > return SCAN_FAIL;
> > > >
> > > > Hmm but in shmem_init() we have:
> > > >
> > > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > > if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY)
> > > > SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge;
> > > > else
> > > > shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */
> > > >
> > > > /*
> > > > * Default to setting PMD-sized THP to inherit the global setting and
> > > > * disable all other multi-size THPs.
> > > > */
> > > > if (!shmem_orders_configured)
> > > > huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER);
> > > > #endif
> > > >
> > > > And shm_mnt->mnt_sb is the superblock used for anon shmem. Also
> > > > shmem_enabled_store() updates that if necessary.
> > > >
> > > > So we're still fine right?
> > > >
> > > > __shmem_file_setup() (used for anon shmem) calls shmem_get_inode() ->
> > > > __shmem_get_inode() which has:
> > > >
> > > > if (sbinfo->huge)
> > > > mapping_set_large_folios(inode->i_mapping);
> > > >
> > > > Shared for both anon shmem and tmpfs-style shmem.
> > > >
> > > > So I think it's fine as-is.
> > >
> > > I'm afraid not. Sorry, I should have been clearer.
> > >
> > > First, anonymous shmem large order allocation is dynamically controlled via
> > > the global interface (/sys/kernel/mm/transparent_hugepage/shmem_enabled) and
> > > the mTHP interfaces
> > > (/sys/kernel/mm/transparent_hugepage/hugepages-*kB/shmem_enabled).
> > >
> > > This means that during anonymous shmem initialization, these interfaces
> > > might be set to 'never'. so it will not call mapping_set_large_folios()
> > > because sbinfo->huge is 'SHMEM_HUGE_NEVER'.
> > >
> > > Even if shmem large order allocation is subsequently enabled via the
> > > interfaces, __shmem_file_setup -> mapping_set_large_folios() is not called
> > > again.
> >
> > I see your point, oh this is all a bit of a mess...
> >
> > It feels like entirely the wrong abstraction anyway, since at best you're
> > getting a global 'is enabled'.
> >
> > I guess what happened before was we'd never call into this with ! r/o thp for fs
> > && ! is_shmem.
>
> Right.
>
> > But now we are allowing it, but should STILL be gating on !is_shmem so yeah your
> > suggestion is correct I think actualyl.
> >
> > I do hate:
> >
> > if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER)
> >
> > As a bit of code though. It's horrible.
>
> Indeed.
>
> > Let's abstract that...
> >
> > It'd be nice if we could find a way to clean things up in the lead up to changes
> > in series like this instead of sticking with the mess, but I guess since it
> > mostly removes stuff that's ok for now.
>
> I think this check can be removed from this patch.
>
> During the khugepaged's scan, it will call thp_vma_allowable_order() to
> check if the VMA is allowed to collapse into a PMD.
>
> Specifically, within the call chain thp_vma_allowable_order() ->
> __thp_vma_allowable_orders(), shmem is checked via
> shmem_allowable_huge_orders(), while other FSes are checked via
> file_thp_enabled().

It sucks not to have an assert. Maybe in that case make it a
VM_WARN_ON_ONCE().

I hate that you're left tracing things back like that...

>
> For those other filesystems, Patch 5 has already added the following check,
> which I think is sufficient to filter out those FSes that do not support
> large folios:
>
> if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER)
> return false;

2 < 5, we won't tolerate bisection hazards.

>
>
> > > Anonymous shmem behaves similarly to anonymous pages: it is controlled by
> > > the 'shmem_enabled' interfaces and uses shmem_allowable_huge_orders() to
> > > check for allowed large orders, rather than relying on
> > > mapping_max_folio_order().
> > >
> > > The mapping_max_folio_order() is intended to control large page allocation
> > > only for tmpfs mounts. Therefore, I find the current code confusing and
> > > think it needs to be fixed:
> > >
> > > /* Don't consider 'deny' for emergencies and 'force' for testing */
> > > if (sb != shm_mnt->mnt_sb && sbinfo->huge)
> > > mapping_set_large_folios(inode->i_mapping);
> >
> > Cheers, Lorenzo
>

Cheers, Lorenzo