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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/14/26 9:25 PM, David Hildenbrand (Arm) wrote:
> 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)) {
> 

Yes, I've considered that. However, implicit memory barriers give me a 
headache, so I ended up going with explicit memory barriers (suggested 
by Pedro).

If you don't like memory barriers, let me re-check whether the implicit 
memory barriers here can ensure race safety with 
sync_with_folio_pmd_zap(). A simplified logic is as follows:

CPU 0 (truncation)                        CPU 1 (zapping)
down_read() ->Acquire A1.                pte spin_lock() ->Acquire A4
                                          clear mapcount

                                          pmd spin_lock() ->Acquire A5
                                          clear pmd
                                          pmd spin_unlock() ->Release R5

if (pmd_none()) { //read pmd
                                          pte spin_unlock() ->Release R4
     pmd spin_lock() ->Acquire A2
     pmd spin_unlock() ->Release R2
}
up_read() -> Release R1

spin_lock(y) ->Acquire A3
folio_mapped() // read mapcount
spin_unlock(y) ->Release R3

CPU 0 constraints:
1. R2 is a release, so "read pmd" completes before R2.
2. A2 completes before A3 (ACQUIRE vs ACQUIRE).
3. "read mapcount" completes after A3, hence after A2.

CPU 1 constraints:
1. R5 is a release. Both "clear mapcount" and "clear pmd" complete 
before R5.

According to above constraints, if CPU0 sees pmd_none() caused by CPU1's 
clear pmd, and CPU0 then acquires the same pmd lock released by CPU1 at 
R5, then CPU1's clear mapcount happens before CPU0's later folio_mapped().

Therefore folio_mapped() is guaranteed not to read the old mapcount in 
this case. So I think sync_with_folio_pmd_zap() can work, but needs a 
small change to avoid the double pmd_none() check.

David, does the following diff look good to you?

diff --git a/mm/memory.c b/mm/memory.c
index 6a8e7772b8d6..c0aadab1b59e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2036,6 +2036,15 @@ static unsigned long zap_pte_range(struct 
mmu_gather *tlb,
                 }
         } while (pte += nr, addr += PAGE_SIZE * nr, addr != end);

+       add_mm_rss_vec(mm, rss);
+       lazy_mmu_mode_disable();
+
+       /* Do the actual TLB flush before dropping ptl */
+       if (force_flush) {
+               tlb_flush_mmu_tlbonly(tlb);
+               tlb_flush_rmaps(tlb, vma);
+       }
+
         /*
          * Fast path: try to hold the pmd lock and unmap the PTE page.
          *
@@ -2044,16 +2053,8 @@ static unsigned long zap_pte_range(struct 
mmu_gather *tlb,
          * to ensure they are still none, thereby preventing the pte 
entries
          * from being repopulated by another thread.
          */
-       if (can_reclaim_pt && direct_reclaim && addr == end)
+       if (can_reclaim_pt && direct_reclaim && addr == end) {
                 direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, 
&pmdval);
-
-       add_mm_rss_vec(mm, rss);
-       lazy_mmu_mode_disable();
-
-       /* Do the actual TLB flush before dropping ptl */
-       if (force_flush) {
-               tlb_flush_mmu_tlbonly(tlb);
-               tlb_flush_rmaps(tlb, vma);
         }
         pte_unmap_unlock(start_pte, ptl);

@@ -2102,12 +2103,11 @@ 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)) {
-                       sync_with_folio_pmd_zap(tlb->mm, pmd);
                 }
+
                 if (pmd_none(*pmd)) {
+                       if (details && details->single_folio)
+                               sync_with_folio_pmd_zap(tlb->mm, pmd);
                         addr = next;
                         continue;
                 }