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Date:   Tue, 15 Aug 2023 08:41:45 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     David Hildenbrand <david@redhat.com>
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Subject: Re: [BUG] Re: [PATCH v3 10/13] mm/khugepaged: collapse_pte_mapped_thp()
 with mmap_read_lock()
In-Reply-To: <76e6b2ad-4e1e-2ad3-95df-00b4d33ec9d2@redhat.com>
Message-ID: <35d52c74-9ba9-211e-a4f-ade6ed318e76@google.com>
References: <7cd843a9-aa80-14f-5eb2-33427363c20@google.com> <b53be6a4-7715-51f9-aad-f1347dcb7c4@google.com> <CAG48ez0FxiRC4d3VTu_a9h=rg5FW-kYD5Rg5xo_RDBM0LTTqZQ@mail.gmail.com> <cacd4a19-386d-8bea-400-e99778dbc3b@google.com>
 <76e6b2ad-4e1e-2ad3-95df-00b4d33ec9d2@redhat.com>
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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/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