Re: [PATCH v2] mm: memory: fix truncation removing mapped folios
From: Baolin Wang
Date: Thu Oct 15 2026 - 02:42:01 EST
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;
}