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

From: Hugh Dickins <[email-masked]>
Date: Mon, 14 Aug 2023 23:34:05 -0700 (PDT)

On Mon, 14 Aug 2023, Jann Horn wrote:
> On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <[email-masked]> 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.

Hugh