Re: [BUG] Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp() with mmap_read_lock()

[David Hildenbrand <david@redhat.com>]

On 15.08.23 08:34, Hugh Dickins wrote:
> On Mon, 14 Aug 2023, Jann Horn wrote:
>> On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <hughd@google.com> wrote:
>>> Bring collapse_and_free_pmd() back into collapse_pte_mapped_thp().
>>> It does need mmap_read_lock(), but it does not need mmap_write_lock(),
>>> nor vma_start_write() nor i_mmap lock nor anon_vma lock.  All racing
>>> paths are relying on pte_offset_map_lock() and pmd_lock(), so use those.
>>
>> We can still have a racing userfaultfd operation at the "/* step 4:
>> remove page table */" point that installs a new PTE before the page
>> table is removed.
>>
>> To reproduce, patch a delay into the kernel like this:
>>
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 9a6e0d507759..27cc8dfbf3a7 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -20,6 +20,7 @@
>>   #include <linux/swapops.h>
>>   #include <linux/shmem_fs.h>
>>   #include <linux/ksm.h>
>> +#include <linux/delay.h>
>>
>>   #include <asm/tlb.h>
>>   #include <asm/pgalloc.h>
>> @@ -1617,6 +1618,11 @@ int collapse_pte_mapped_thp(struct mm_struct
>> *mm, unsigned long addr,
>>          }
>>
>>          /* step 4: remove page table */
>> +       if (strcmp(current->comm, "DELAYME") == 0) {
>> +               pr_warn("%s: BEGIN DELAY INJECTION\n", __func__);
>> +               mdelay(5000);
>> +               pr_warn("%s: END DELAY INJECTION\n", __func__);
>> +       }
>>
>>          /* Huge page lock is still held, so page table must remain empty */
>>          pml = pmd_lock(mm, pmd);
>>
>>
>> And then run the attached reproducer against mm/mm-everything. You
>> should get this in dmesg:
>>
>> [  206.578096] BUG: Bad rss-counter state mm:000000000942ebea
>> type:MM_ANONPAGES val:1
> 
> Thanks a lot, Jann. I haven't thought about it at all yet; and just
> tried to reproduce, but haven't yet got the "BUG: Bad rss-counter":
> just see "Invalid argument" on the UFFDIO_COPY ioctl.
> Will investigate tomorrow.

Maybe you're missing a fixup:

https://lkml.kernel.org/r/20230810192128.1855570-1-axelrasmussen@google.com

When the src address is not page aligned, UFFDIO_COPY in mm-unstable 
would erroneously fail.

-- 
Cheers,

David / dhildenb