Re: [PATCH v2] mm: memory: fix truncation removing mapped folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 10/13/26 11:36, Baolin Wang wrote:
> 
> 
> On 10/12/26 6:23 PM, David Hildenbrand (Arm) wrote:
>> On 10/10/26 09:22, Baolin Wang wrote:
>>> Ayush reported an issue that shmem/tmpfs folios are still mapped when
>>> truncating them on production:
>>> "
>>> BUG: Bad page cache in process ...  pfn:...
>>> page dumped because: still mapped when deleted
>>> "
>>>
>>> I reproduced the issue with Ayush's reproducer [1] with the latest mm-unstable
>>> branch, and I got the following crash info. From the dump message, we can
>>> see that truncate_inode_folio() is really trying to remove mapped folios,
>>> which is incorrect.
>>>
>>> "
>>> [  189.171977] page: refcount:3 mapcount:1 mapping:00000000fe168ee1
>>> index:0x3880 pfn:0x191647
>>> [  189.171995] memcg:ffff0000cc073d40
>>> [  189.171997] aops:shmem_aops ino:3c01 dentry name(?):"memfd:runsc-memory"
>>> [  189.172006] flags: 0x17fffef0002022d(locked|referenced|uptodate|lru|
>>> workingset|swapbacked|node=0|zone=2|lastcpupid=0x3ffff)
>>> .......
>>> [  189.172017] page dumped because: VM_BUG_ON_FOLIO(folio_mapped(folio))
>>> [  189.172026] ------------[ cut here ]------------
>>> [  189.172027] kernel BUG at mm/filemap.c:155!
>>> [  189.172057] Internal error: Oops - BUG: 00000000f2000800 [#1]  SMP
>>> .......
>>> [  189.177721] CPU: 3 UID: 0 PID: 5521 Comm: repro Kdump: loaded Tainted:
>>> 7.3.0-rc4+ #277 PREEMPT(lazy)
>>> .......
>>> [  189.183986] Call trace:
>>> [  189.184124]  filemap_unaccount_folio+0xf0/0x1e8 (P)
>>> [  189.184391]  __filemap_remove_folio+0x34/0x160
>>> [  189.184633]  filemap_remove_folio+0x4c/0xb0
>>> [  189.184859]  truncate_inode_folio+0x34/0x58
>>> [  189.185087]  shmem_undo_range+0x220/0x658
>>> [  189.185308]  shmem_fallocate+0x2f0/0x470
>>> [  189.185527]  vfs_fallocate+0x128/0x328
>>> ......
>>> [  189.187078]  el0t_64_sync+0x184/0x188
>>> "
>>>
>>> After analysis, I believe the race exists between truncation and MADV_DONTNEED,
>>> and shmem's ->map_pages() merely makes the issue easier to reproduce. Since
>>> MADV_DONTNEED synchronously releases the pagetable page before calling
>>> tlb_flush_rmaps(), this could cause another thread's truncation to skip
>>> zap_pte_range() (due to pmd is none) but still observe the folio's mapcount
>>> as non-zero. A possible race scenario is as follows:
>>>
>>> CPU 0                           CPU 1
>>> shmem_fallocate                 madvise_dontneed_single_vma
>>>    unmap_mapping_range            .......
>>>    ......
>>>    (filemap_map_pages() remap folios)
>>>                   zap_pte_range
>>>    truncate_inode_folio            zap_empty_pte_table(pmd clear)
>>>      unmap_mapping_folio
>>>      ......
>>>        zap_pmd_range (saw pmd none, skip)
>>>
>>>    filemap_remove_folio
>>>      BUG_ON(folio_mapped)
>>
>> That reminds me of sync_with_folio_pmd_zap() usage in page_vma_mapped_walk().
>>
>> Isn't that the better approach?
> 
> I had considered this approach before, but it is not feasible. IIUC, the
> sync_with_folio_pmd_zap() is used to address the race with zap_huge_pmd(), and
> both the PMD clear and the folio mapcount decrement are performed under the PMD
> lock in zap_huge_pmd().

Just to be clear, I am thinking about something like the following on the
zap_pmd_range() side:

diff --git a/mm/internal.h b/mm/internal.h
index 38b1165212c94..a8820ad7e2f21 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -584,14 +584,15 @@ void free_pgtables(struct mmu_gather *tlb, struct
unmap_desc *desc);
 void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);

 /**
- * sync_with_folio_pmd_zap - sync with concurrent zapping of a folio PMD
+ * sync_with_folio_pmd_zap - sync with concurrent zapping of folios
  * @mm: The mm_struct.
  * @pmdp: Pointer to the pmd that was found to be pmd_none().
  *
  * When we find a pmd_none() while unmapping a folio without holding the PTL,
- * zap_huge_pmd() may have cleared the PMD but not yet modified the folio to
- * indicate that it's unmapped. Skipping the PMD without synchronization could
- * make folio unmapping code assume that unmapping failed.
+ * zap_huge_pmd() or zap_empty_pte_table() may have cleared the PMD but not
+ * yet modified the mapped folios to indicate they are unmapped. Skipping the
+ * PMD without synchronization could make folio unmapping code assume that
+ * unmapping failed.
  *
  * Wait for concurrent zapping to complete by grabbing the PTL.
  */
diff --git a/mm/memory.c b/mm/memory.c
index 8b0c2c735d3de..c585f5eb255c9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2139,9 +2139,7 @@ static inline unsigned long zap_pmd_range(struct
mmu_gather *tlb,
                                continue;
                        }
                        /* fall through */
-               } else if (details && details->single_folio &&
-                          folio_test_pmd_mappable(details->single_folio) &&
-                          next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) {
+               } else if (details && details->single_folio && pmd_none(*pmd)) {
                        sync_with_folio_pmd_zap(tlb->mm, pmd);
                }
                if (pmd_none(*pmd)) {

> 
> However, the race here is different: tlb_flush_rmaps() is under the PTE lock,
> while zap_empty_pte_table() is under the PMD lock, and reordering between the
> two functions is also possible. That is, the folio mapcount operation could
> execute after the acquire barrier (spin_lock()) of zap_empty_pte_table(),
> allowing a reader on CPU 0 to incorrectly observe that the PMD is none while
> mapcount > 0.

I think we really just have to make sure to flush rmaps before zapping the PMD.
Then it's really just the same thing we had before with sync_with_folio_pmd_zap().

So I agree about doing the tlb_flush_rmaps() before zapping the PMD. But on the
zap_pmd_range() front that's really just what sync_with_folio_pmd_zap() wants to
handle.

-- 
Cheers,

David