Re: [PATCH v5 1/1] mm/rmap: fix soft-dirty and uffd-wp bit loss when remapping zero-filled mTHP subpage to shared zeropage

[Lance Yang <lance.yang@linux.dev>]

Thanks for the super energetic review!

On 2025/10/14 19:19, Lorenzo Stoakes wrote:
> Feels like the mTHP implementation is hitting up on the buffers of the THP code
> being something of a mess... :)

Haha, yeah, it really feels that way sometimes ;)

> 
> On Tue, Sep 30, 2025 at 04:10:40PM +0800, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> When splitting an mTHP and replacing a zero-filled subpage with the shared
>> zeropage, try_to_map_unused_to_zeropage() currently drops several important
>> PTE bits.
>>
>> For userspace tools like CRIU, which rely on the soft-dirty mechanism for
> 
> It's slightly by-the-by, but CRIU in my view - as it relies on kernel
> implementation details that can always change to operate - is not actually
> something we have to strictly keep working.
> 
> HOWEVER, if we can reasonably do so without causing issues for us in the kernel
> we ought to do so.
> 
>> incremental snapshots, losing the soft-dirty bit means modified pages are
>> missed, leading to inconsistent memory state after restore.
>>
>> As pointed out by David, the more critical uffd-wp bit is also dropped.
>> This breaks the userfaultfd write-protection mechanism, causing writes
>> to be silently missed by monitoring applications, which can lead to data
>> corruption.
> 
> Again, uffd-wp is a total mess. We shouldn't be in a position where its state
> being correctly retained relies on everybody always getting the subtle,
> uncommented, open-coded details right everywhere all the time.
> 
> But this is again a general comment... :)

:)

> 
>>
>> Preserve both the soft-dirty and uffd-wp bits from the old PTE when
>> creating the new zeropage mapping to ensure they are correctly tracked.
>>
>> Cc: <stable@vger.kernel.org>
>> Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
>> Suggested-by: David Hildenbrand <david@redhat.com>
>> Suggested-by: Dev Jain <dev.jain@arm.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Reviewed-by: Dev Jain <dev.jain@arm.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
> 
> Overall LGTM, so:
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Cheers!

> 
>> ---
>> v4 -> v5:
>>   - Move ptep_get() call after the !pvmw.pte check, which handles PMD-mapped
>>     THP migration entries.
>>   - https://lore.kernel.org/linux-mm/20250930071053.36158-1-lance.yang@linux.dev/
>>
>> v3 -> v4:
>>   - Minor formatting tweak in try_to_map_unused_to_zeropage() function
>>     signature (per David and Dev)
>>   - Collect Reviewed-by from Dev - thanks!
>>   - https://lore.kernel.org/linux-mm/20250930060557.85133-1-lance.yang@linux.dev/
>>
>> v2 -> v3:
>>   - ptep_get() gets called only once per iteration (per Dev)
>>   - https://lore.kernel.org/linux-mm/20250930043351.34927-1-lance.yang@linux.dev/
>>
>> v1 -> v2:
>>   - Avoid calling ptep_get() multiple times (per Dev)
>>   - Double-check the uffd-wp bit (per David)
>>   - Collect Acked-by from David - thanks!
>>   - https://lore.kernel.org/linux-mm/20250928044855.76359-1-lance.yang@linux.dev/
>>
>>   mm/migrate.c | 15 ++++++++++-----
>>   1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index ce83c2c3c287..e3065c9edb55 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -296,8 +296,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list)
>>   }
>>
>>   static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>> -					  struct folio *folio,
>> -					  unsigned long idx)
>> +		struct folio *folio, pte_t old_pte, unsigned long idx)
>>   {
>>   	struct page *page = folio_page(folio, idx);
>>   	pte_t newpte;
>> @@ -306,7 +305,7 @@ static bool try_to_map_unused_to_zeropage(struct page_vma_mapped_walk *pvmw,
>>   		return false;
>>   	VM_BUG_ON_PAGE(!PageAnon(page), page);
>>   	VM_BUG_ON_PAGE(!PageLocked(page), page);
>> -	VM_BUG_ON_PAGE(pte_present(ptep_get(pvmw->pte)), page);
>> +	VM_BUG_ON_PAGE(pte_present(old_pte), page);
> 
> Kinda ugly that we pass old_pte when it's avaiable via the shared state object,
> but probably nothing to really concern ourselves about.
> 
> Guess you could argue it both ways :)
> 
> It'd be good to convert these VM_BUG_ON_*() to VM_WARN_ON_*() but I guess that's

I agree.

> somewhat out of the scope of the code here and would be inconsistent to change
> it for just one condition.

Since this fix already landed in the mainline, just leave it as is here :)

Thanks,
Lance