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

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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:

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


David / dhildenb

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