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

[David Hildenbrand]

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@xxxxxxxxxx> 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@xxxxxxxxxx

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

--
Cheers,

David / dhildenb