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 <[email protected]> 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/[email protected]
When the src address is not page aligned, UFFDIO_COPY in mm-unstable
would erroneously fail.
--
Cheers,
David / dhildenb
On Tue, 15 Aug 2023, David Hildenbrand wrote:
> On 15.08.23 08:34, Hugh Dickins wrote:
> > On Mon, 14 Aug 2023, Jann Horn wrote:
> >>
> >> /* 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/[email protected]
>
> When the src address is not page aligned, UFFDIO_COPY in mm-unstable would
> erroneously fail.
You got it, many thanks David: I had assumed that my next-20230808 tree
would be up-to-date enough, but it wasn't. Reproduced now.
Hugh