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

[Hugh Dickins <hughd@google.com>]

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On Mon, 14 Aug 2023, Jann Horn wrote:
> On Wed, Jul 12, 2023 at 6:42 AM Hugh Dickins <hughd@google.com> 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.

And you've been very polite not to remind me that this is exactly
what you warned me about, in connection with retract_page_tables(),
nearly three months ago:

https://lore.kernel.org/linux-mm/CAG48ez0aF1Rf1apSjn9YcnfyFQ4YqSd4GqB6f2wfhF7jMdi5Hg@mail.gmail.com/

> 
> 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

Very helpful, thank you Jann.

I got a bit distracted when I then found mm's recent addition of
UFFDIO_POISON: thought I needed to change both collapse_pte_mapped_thp()
and retract_page_tables() now to cope with mfill_atomic_pte_poison()
inserting into even a userfaultfd_armed shared VMA.

But eventually, on second thoughts, realized that's only inserting a pte
marker, invalid, so won't cause any actual trouble.  A little untidy,
to leave that behind in a supposedly empty page table about to be freed,
but not worth refactoring these functions to avoid a non-bug.

And though syzbot and JH may find some fun with it, I don't think any
real application would be insertng a PTE_MARKER_POISONED where a huge
page collapse is almost complete.

So I scaled back to a more proportionate fix, following.  Sorry, I've
slightly messed up applying the "DELAY INJECTION" patch above: not
intentional, honest!  (mdelay while holding the locks is still good.)

Hugh