Re: [Patch mm-hotfixes v4] mm/page_vma_mapped: fix device-private PMD handling

[Lorenzo Stoakes]

On Fri, Jun 26, 2026 at 09:24:06AM -0400, Zi Yan wrote:
> On 26 Jun 2026, at 7:31, David Hildenbrand (Arm) wrote:
> 
> > On 6/26/26 12:42, Lorenzo Stoakes wrote:
> >> On Fri, Jun 26, 2026 at 12:07:56PM +0200, David Hildenbrand (Arm) wrote:
> >>> On 6/24/26 08:53, Wei Yang wrote:
> >>>> Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support
> >>>> device-private entries") introduced the concept of device-private
> >>>> PMD entries, but did not correctly update the rmap walk code to
> >>>> account for them.
> >>>>
> >>>> As a result, when page_vma_mapped_walk() encounters device-private
> >>>> PMD entries, it takes no action other than to acquire the PMD lock
> >>>> and exit.
> >>>>
> >>>> However this is highly problematic for two reasons - firstly,
> >>>> device private entries possess a PFN so check_pmd() needs to be
> >>>> called to ensure an overlapping PFN range.
> >>>>
> >>>> Secondly, and more importantly, if PVMW_MIGRATION is set the
> >>>> caller assumes the returned entry is a migration entry, resulting
> >>>> in memory corruption when the caller tries to interpret the device
> >>>> private entry as such.
> >>>>
> >>>> In addition, commit 146287290023 ("mm/huge_memory: implement
> >>>> device-private THP splitting") allowed device private PMDs to be
> >>>> split like THP mappings, but again did not update this code path.
> >>>>
> >>>> As a result, we might race a PMD split prior to acquiring the PMD
> >>>> lock.
> >>>>
> >>>> This patch addresses all of these issues by invoking check_pmd(),
> >>>> ensuring PMVW_MIGRATION is not set and checks whether a split raced
> >>>> us we do for PMD THP and migration entries.
> >>>>
> >>>> Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries")
> >>>> Cc: <stable@xxxxxxxxxxxxxxx>
> >>>> Signed-off-by: Wei Yang <richard.weiyang@xxxxxxxxx>
> >>>> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> >>>> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >>>> Cc: Balbir Singh <balbirs@xxxxxxxxxx>
> >>>> Cc: SeongJae Park <sj@xxxxxxxxxx>
> >>>> Cc: Zi Yan <ziy@xxxxxxxxxx>
> >>>> Cc: Lorenzo Stoakes <ljs@xxxxxxxxxx>
> >>>> Cc: Lance Yang <lance.yang@xxxxxxxxx>
> >>>>
> >>>> ---
> >>>> v4:
> >>>>   * refine subject and commit log based on Lorenzo's suggestion
> >>>>   * put pmd device-private entry handling in its own if branch,
> >>>>     suggested by Lorenzo
> >>>>
> >>>> v3:
> >>>>   * remove cleanup part, only fix the issue for device-private entry
> >>>>   * refine user effect description based on Lorenzo's suggestion
> >>>>
> >>>> v2: https://lore.kernel.org/all/20260616063436.20455-1-richard.weiyang@xxxxxxxxx/T/#u
> >>>>   * specify the possible error case of current code and user visible effect
> >>>>   * besides fix, cleanup the pmd entry handling based on David's suggestion
> >>>>
> >>>> v1: https://lore.kernel.org/linux-mm/20260508013728.21285-1-richard.weiyang@xxxxxxxxx/
> >>>> ---
> >>>>  mm/page_vma_mapped.c | 20 +++++++++++++++-----
> >>>>  1 file changed, 15 insertions(+), 5 deletions(-)
> >>>>
> >>>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> >>>> index 2ccbabfb2cc1..17dff8aab9f9 100644
> >>>> --- a/mm/page_vma_mapped.c
> >>>> +++ b/mm/page_vma_mapped.c
> >>>> @@ -269,14 +269,24 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> >>>>  			/* THP pmd was split under us: handle on pte level */
> >>>>  			spin_unlock(pvmw->ptl);
> >>>>  			pvmw->ptl = NULL;
> >>>> -		} else if (!pmd_present(pmde)) {
> >>>> -			const softleaf_t entry = softleaf_from_pmd(pmde);
> >>>> +		} else if (pmd_is_device_private_entry(pmde)) {
> >>>> +			softleaf_t entry;
> >>>> +
> >>>> +			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> >>>> +			pmde = *pvmw->pmd;
> >>>> +			entry = softleaf_from_pmd(pmde);
> >>>>
> >>>> -			if (softleaf_is_device_private(entry)) {
> >>>> -				pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> >>>> +			if (likely(softleaf_is_device_private(entry))) {
> >>>> +				if (pvmw->flags & PVMW_MIGRATION)
> >>>> +					return not_found(pvmw);
> >>>> +				if (!check_pmd(softleaf_to_pfn(entry), pvmw))
> >>>> +					return not_found(pvmw);
> >>>>  				return true;
> >>>>  			}
> >>>> -
> >>>> +			/* device-private pmd was split under us: handle on pte level */
> >>>> +			spin_unlock(pvmw->ptl);
> >>>> +			pvmw->ptl = NULL;
> >>>> +		} else if (!pmd_present(pmde)) {
> >>>>  			if ((pvmw->flags & PVMW_SYNC) &&
> >>>>  			    thp_vma_suitable_order(vma, pvmw->address,
> >>>>  						   PMD_ORDER) &&
> >>>
> >>> This is extremely hard to review given the existing crap handling here. I'm
> >>> really sorry, but it makes my head hurt (I'm not kidding :) ).
> >>>
> >>> It's completely unclear why we only have to check for a subset of the cases
> >>> after taking the lock.
> >>>
> >>> Could we simply extend the existing migration pmd handling and leave the
> >>> !pmd_present() case for pmd_none()?
> >>>
> >>> That leaves no question to "which transitions are actually allowed", including
> >>> "could we accidentally assume something is a page table when really it isn't".
> >>>
> >>>
> >>> So what about something like the following?
> >>>
> >>> The "thp_migration_supported()" is not required when checking for
> >>> pmd_is_migration_entry(), as that defaults to "false" when not compiled in.
> >>>
> >>> Untested:
> >>>
> >>>
> >>> From 048ecd33673ec649e168fbbb97749a7c0e344fcd Mon Sep 17 00:00:00 2001
> >>> From: "David Hildenbrand (Arm)" <david@xxxxxxxxxx>
> >>> Date: Fri, 26 Jun 2026 12:03:40 +0200
> >>> Subject: [PATCH] tmp
> >>>
> >>> Signed-off-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> >>> ---
> >>>  mm/page_vma_mapped.c | 29 +++++++++++++++++------------
> >>>  1 file changed, 17 insertions(+), 12 deletions(-)
> >>>
> >>> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> >>> index 2ccbabfb2cc17..ed2a23a90e8dd 100644
> >>> --- a/mm/page_vma_mapped.c
> >>> +++ b/mm/page_vma_mapped.c
> >>> @@ -243,21 +243,31 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> >>>  		 */
> >>>  		pmde = pmdp_get_lockless(pvmw->pmd);
> >>>
> >>> -		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde)) {
> >>> +		if (pmd_trans_huge(pmde) || pmd_is_migration_entry(pmde) ||
> >>> +		    pmd_is_device_private_entry(pmde)) {
> >>>  			pvmw->ptl = pmd_lock(mm, pvmw->pmd);
> >>>  			pmde = *pvmw->pmd;
> >>> -			if (!pmd_present(pmde)) {
> >>> +			if (pmd_is_migration_entry(pmde)) {
> >>>  				softleaf_t entry;
> >>>
> >>> -				if (!thp_migration_supported() ||
> >>
> >> Do we care about this? Or is !tmp_migration_supported() -> implies you
> >> wouldn't see a migration entry here anyway?
> >
> > Yeah, I noted above
> >
> > "The "thp_migration_supported()" is not required when checking for
> > pmd_is_migration_entry(), as that defaults to "false" when not compiled in."
> >
> > Given that
> >
> > tmp_migration_supported() -> IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION);$
> >
> > And
> >
> > pmd_is_migration_entry() -> softleaf_is_migration(softleaf_from_pmd(pmd));
> >
> > whereby softleaf_from_pmd() only returns something non-none for
> > CONFIG_ARCH_ENABLE_THP_MIGRATION.
> >
> >>
> >> Maybe worth a VM_WARN_ON_ONCE()?
> >
> > I think it was primarily a a hack to slightly optimize code generated for
> > !CONFIG_ARCH_ENABLE_THP_MIGRATION, not really something for correctness as it seems.
> >
> > So I think we can safely drop it. :)
> 
> thp_migration_supported() here is legacy code[1] from v4.14 when I added
> the THP migration support. IIRC, the purpose was to avoid checking
> PMD migration entry if the support is not enabled, but looking at it again
> today, that thp_migration_supported() is unnecessary since
> is_migration_entry(pmd_to_swp_entry(*pvmw->pmd)) returns false if
> !CONFIG_ARCH_ENABLE_THP_MIGRATION.
> 
> [1] https://elixir.bootlin.com/linux/v4.14/source/mm/page_vma_mapped.c#L157

Thanks guys, let's drop it then!

> 
> Best Regards,
> Yan, Zi

Cheers, Lorenzo