Re: [Bug 215804] New: [xfstests generic/670] Unable to handle kernel paging request at virtual address fffffbffff000008

[Matthew Wilcox <willy@infradead.org>]

On Tue, Apr 05, 2022 at 04:44:35AM +0000, bugzilla-daemon@kernel.org wrote:
> https://bugzilla.kernel.org/show_bug.cgi?id=215804
[...]
> [37285.232165] Unable to handle kernel paging request at virtual address
> fffffbffff000008 
> [37285.232776] KASAN: maybe wild-memory-access in range
> [0x0003dffff8000040-0x0003dffff8000047] 
> [37285.233332] Mem abort info: 
> [37285.233520]   ESR = 0x96000006 
> [37285.233725]   EC = 0x25: DABT (current EL), IL = 32 bits 
> [37285.234077]   SET = 0, FnV = 0 
> [37285.234281]   EA = 0, S1PTW = 0 
> [37285.234544]   FSC = 0x06: level 2 translation fault 
> [37285.234871] Data abort info: 
> [37285.235065]   ISV = 0, ISS = 0x00000006 
> [37285.235319]   CM = 0, WnR = 0 
> [37285.235517] swapper pgtable: 4k pages, 48-bit VAs, pgdp=00000004574eb000 
> [37285.235953] [fffffbffff000008] pgd=0000000458c71003, p4d=0000000458c71003,
> pud=0000000458c72003, pmd=0000000000000000 
> [37285.236651] Internal error: Oops: 96000006 [#1] SMP 
> [37285.239187] CPU: 3 PID: 3302514 Comm: xfs_io Kdump: loaded Tainted: G       W         5.17.0+ #1 
> [37285.239810] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015 
> [37285.240292] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) 
> [37285.240783] pc : __split_huge_pmd+0x1d8/0x34c 
> [37285.241097] lr : __split_huge_pmd+0x174/0x34c 
> [37285.241407] sp : ffff800023a56fe0 
> [37285.241642] x29: ffff800023a56fe0 x28: 0000000000000000 x27:
> ffff0001c54d4060 
> [37285.242145] x26: 0000000000000000 x25: 0000000000000000 x24:
> fffffc00056cf000 
> [37285.242661] x23: 1ffff0000474ae0a x22: ffff0007104fe630 x21:
> ffff00014fab66b0 
> [37285.243175] x20: ffff800023a57080 x19: fffffbffff000000 x18:
> 0000000000000000 
> [37285.243689] x17: 0000000000000000 x16: ffffb109a2ec7e30 x15:
> 0000ffffd9035c10 
> [37285.244202] x14: 00000000f2040000 x13: 0000000000000000 x12:
> ffff70000474aded 
> [37285.244715] x11: 1ffff0000474adec x10: ffff70000474adec x9 :
> dfff800000000000 
> [37285.245230] x8 : ffff800023a56f63 x7 : 0000000000000001 x6 :
> 0000000000000003 
> [37285.245745] x5 : ffff800023a56f60 x4 : ffff70000474adec x3 :
> 1fffe000cd086e01 
> [37285.246257] x2 : 1fffff7fffe00001 x1 : 0000000000000000 x0 :
> fffffbffff000008 
> [37285.246770] Call trace: 
> [37285.246952]  __split_huge_pmd+0x1d8/0x34c 
> [37285.247246]  split_huge_pmd_address+0x10c/0x1a0 
> [37285.247577]  try_to_unmap_one+0xb64/0x125c 
> [37285.247878]  rmap_walk_file+0x1dc/0x4b0 
> [37285.248159]  try_to_unmap+0x134/0x16c 
> [37285.248427]  split_huge_page_to_list+0x5ec/0xcbc 
> [37285.248763]  truncate_inode_partial_folio+0x194/0x2ec 

Clearly this is due to my changes, but I'm wondering why it doesn't
happen with misaligned mappings and shmem today.  Here's the path I
see as being problematic:

split_huge_page()
  split_huge_page_to_list()
    unmap_page()
      ttu_flags = ... TTU_SPLIT_HUGE_PMD ...
      try_to_unmap()
        try_to_unmap_one()
	  split_huge_pmd_address()
	    pmd = pmd_offset(pud, address);
	    __split_huge_pmd(vma, pmd, address, freeze, folio);
	      if (folio) {
                if (folio != page_folio(pmd_page(*pmd)))

I'm assuming it's crashing at that line.  Calling pmd_page() on a
pmd that we haven't checked is pmd_trans_huge() seems like a really
bad idea.  I probably compounded that problem by calling page_folio()
on something that's not necessarily a PMD that points to a page, but
I think the real sin here is that nobody checks before this that it's
trans_huge.

Here's Option A for fixing it: Only check pmd_page() after checking
pmd_trans_huge():

+++ b/mm/huge_memory.c
@@ -2145,15 +2145,14 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
         * pmd against. Otherwise we can end up replacing wrong folio.
         */
        VM_BUG_ON(freeze && !folio);
-       if (folio) {
-               VM_WARN_ON_ONCE(!folio_test_locked(folio));
-               if (folio != page_folio(pmd_page(*pmd)))
-                       goto out;
-       }
+       VM_WARN_ON_ONCE(folio && !folio_test_locked(folio));

        if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd) ||
-           is_pmd_migration_entry(*pmd))
+           is_pmd_migration_entry(*pmd)) {
+               if (folio && folio != page_folio(pmd_page(*pmd)))
+                       goto out;
                __split_huge_pmd_locked(vma, pmd, range.start, freeze);
+       }

 out:
        spin_unlock(ptl);

I can think of a few more ways of fixing it, but that one seems best.
Not tested in any meaningful way, more looking for feedback.