On Tue, 15 Aug 2023, David Hildenbrand wrote:

On 15.08.23 08:34, Hugh Dickins wrote:

quoted

On Mon, 14 Aug 2023, Jann Horn wrote:

quoted

         /* 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.

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