Re: [PATCH v4] mm: memory: fix truncation removing mapped folios

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

On 10/26/26 03:05, 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)
>                                     tlb_flush_rmaps(remove rmaps)
> 
> To fix this issue, we should move the pmd clear operation (via zap_empty_pte_table())
> to after tlb_flush_rmaps() and use sync_with_folio_pmd_zap() to sync with
> concurrent zapping, so that when we find a pmd_none() while unmapping a folio
> without holding the PTL, the folio's mappings are guaranteed to have been
> removed via tlb_flush_rmaps().
> 
> [1] https://lore.kernel.org/all/20260925065013.3682431-1-ayushr@modal.com/
> Reported-by: Ayush Ranjan <ayushr@modal.com>
> Closes: https://lore.kernel.org/all/20260924061708.1645968-1-ayushr@modal.com/
> Fixes: 6375e95f381e ("mm: pgtable: reclaim empty PTE page in madvise(MADV_DONTNEED)")
> Acked-by: Qi Zheng <qi.zheng@linux.dev>
> Reviewed-by: Pedro Falcato <pfalcato@suse.de>
> Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
> Changes from v3:
>  - Collect review tags from Qi, Pedro and Lorenzo. Thanks.
>  - Update the related comments per Lorenzo.
> Changes from v2:
>  - Use use sync_with_folio_pmd_zap() to sync with concurrent zapping (Per David).
> Changes from v1:
>  - Add an smp_rmb() to pair with the smp_wmb() (Per Lorenzo and Pedro).
> ---
>  mm/internal.h | 10 ++++++----
>  mm/memory.c   | 28 +++++++++++++++-------------
>  2 files changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index b1b0f29d2216..ffb8eaec90e3 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -584,14 +584,16 @@ 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() may have cleared the PMD but not yet updated the folio
> + * mapcount, and zap_empty_pte_table() clears the PMD only after the rmaps
> + * for the table have been removed, but nothing orders the pmd_none() read
> + * against later mapcount reads. 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 e911e63ba1ed..6a93b7fa48b2 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.
>  	 *
> @@ -2043,18 +2052,12 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>  	 * to hold the pmd lock failed, then we need to recheck all pte entries
>  	 * to ensure they are still none, thereby preventing the pte entries
>  	 * from being repopulated by another thread.
> +	 *
> +	 * Clear the PMD after the rmaps for the table have been removed, in case
> +	 * an incorrect mapcount is observed after unmapping the folio.
>  	 */
>  	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 +2105,11 @@ 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;
>  		
Sorry if I sound annoyed, but why don't you keep the

} else if (pmd_none(*pmd)) {

above as I suggested?

Because we fancy unnecessary PMD table lock cycles?


-- 
Cheers,

David