Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs

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

On 2025/9/18 15:37, Dev Jain wrote:
> 
> On 18/09/25 10:34 am, Lance Yang wrote:
>> From: Lance Yang <lance.yang@linux.dev>
>>
>> Guard PTE markers are installed via MADV_GUARD_INSTALL to create
>> lightweight guard regions.
>>
>> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when
>> encountering such a range.
>>
>> MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in
>> the special marker in __collapse_huge_page_swapin().
>>
>> hpage_collapse_scan_pmd()
>>   `- collapse_huge_page()
>>       `- __collapse_huge_page_swapin() -> fails!
>>
>> khugepaged's behavior is slightly different due to its max_ptes_swap 
>> limit
>> (default 64). It won't fail as deep, but it will still needlessly scan up
>> to 64 swap entries before bailing out.
>>
>> IMHO, we can and should detect this much earlier.
>>
>> This patch adds a check directly inside the PTE scan loop. If a guard
>> marker is found, the scan is aborted immediately with 
>> SCAN_PTE_NON_PRESENT,
>> avoiding wasted work.
>>
>> Suggested-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>> Signed-off-by: Lance Yang <lance.yang@linux.dev>
>> ---
>>   mm/khugepaged.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 9ed1af2b5c38..70ebfc7c1f3e 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct 
>> mm_struct *mm,
>>                       result = SCAN_PTE_UFFD_WP;
>>                       goto out_unmap;
>>                   }
>> +                /*
>> +                 * Guard PTE markers are installed by
>> +                 * MADV_GUARD_INSTALL. Any collapse path must
>> +                 * not touch them, so abort the scan immediately
>> +                 * if one is found.
>> +                 */
>> +                if (is_guard_pte_marker(pteval)) {
>> +                    result = SCAN_PTE_NON_PRESENT;
>> +                    goto out_unmap;
>> +                }
>>                   continue;
>>               } else {
>>                   result = SCAN_EXCEED_SWAP_PTE;
>>
>>
> 
> I would like to hear everyone else's thoughts on
> https://lore.kernel.org/linux-mm/750a06dc-db3d-43c6- 
> b234-95efb393a9df@arm.com/
> wherein I suggest that we should not continue to try collapsing other 
> regions
> but immediately exit. The SCAN_PTE_NON_PRESENT case does not exit.

Yes! Let's hear what other folks think on that[1].

[1] 
https://lore.kernel.org/linux-mm/c9d4d761-202f-48ce-8e3d-fb9075671ff3@linux.dev

Cheers,
Lance