Re: [RFC PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()
From: Baolin Wang
Date: Mon Mar 16 2026 - 21:16:39 EST
On 3/16/26 10:06 PM, David Hildenbrand (Arm) wrote:
On 3/13/26 04:45, Baolin Wang wrote:
When running stress-ng on my Arm64 machine with v7.0-rc3 kernel, I encountered
some very strange crash issues showing up as "Bad page state":
"
[ 734.496287] BUG: Bad page state in process stress-ng-env pfn:415735fb
[ 734.496427] page: refcount:0 mapcount:1 mapping:0000000000000000 index:0x4cf316 pfn:0x415735fb
[ 734.496434] flags: 0x57fffe000000800(owner_2|node=1|zone=2|lastcpupid=0x3ffff)
[ 734.496439] raw: 057fffe000000800 0000000000000000 dead000000000122 0000000000000000
[ 734.496440] raw: 00000000004cf316 0000000000000000 0000000000000000 0000000000000000
[ 734.496442] page dumped because: nonzero mapcount
"
After analyzing this page’s state, it is hard to understand why the mapcount
is not 0 while the refcount is 0, since this page is not where the issue first
occurred. By enabling the CONFIG_DEBUG_VM config, I can reproduce the crash as
well and captured the first warning where the issue appears:
"
[ 734.469226] page: refcount:33 mapcount:0 mapping:00000000bef2d187 index:0x81a0 pfn:0x415735c0
[ 734.469304] head: order:5 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
[ 734.469315] memcg:ffff000807a8ec00
[ 734.469320] aops:ext4_da_aops ino:100b6f dentry name(?):"stress-ng-mmaptorture-9397-0-2736200540"
[ 734.469335] flags: 0x57fffe400000069(locked|uptodate|lru|head|node=1|zone=2|lastcpupid=0x3ffff)
......
[ 734.469364] page dumped because: VM_WARN_ON_FOLIO((_Generic((page + nr_pages - 1),
const struct page *: (const struct folio *)_compound_head(page + nr_pages - 1), struct page *:
(struct folio *)_compound_head(page + nr_pages - 1))) != folio)
[ 734.469390] ------------[ cut here ]------------
[ 734.469393] WARNING: ./include/linux/rmap.h:351 at folio_add_file_rmap_ptes+0x3b8/0x468,
CPU#90: stress-ng-mlock/9430
[ 734.469551] folio_add_file_rmap_ptes+0x3b8/0x468 (P)
[ 734.469555] set_pte_range+0xd8/0x2f8
[ 734.469566] filemap_map_folio_range+0x190/0x400
[ 734.469579] filemap_map_pages+0x348/0x638
[ 734.469583] do_fault_around+0x140/0x198
......
[ 734.469640] el0t_64_sync+0x184/0x188
"
The code that triggers the warning is: "VM_WARN_ON_FOLIO(page_folio(page + nr_pages - 1) != folio, folio)",
which indicates that set_pte_range() tried to map beyond the large folio’s
size.
We had a bunch of similar reports throughout the last year from syzbot,
but always without a reproducer.
I can reproduce it by running stress-ng for a few hours on my 128‑core machine, but I can’t reproduce it on smaller machines (e.g., 32‑core systems). It seems we need sufficient concurrency to trigger this race.
In particular this here:
https://syzkaller.appspot.com/bug?extid=c0673e1f1f054fac28c2
via
https://lore.kernel.org/all/6758f0cc.050a0220.17f54a.0001.GAE@xxxxxxxxxx/
Could that be the same root cause?
It appears so. Its warning stack calltrace matches what I reproduced.
By adding more debug information, I found that 'nr_pages' had overflowed in
filemap_map_pages(), causing set_pte_range() to establish mappings for a range
exceeding the folio size, potentially corrupting fields of pages that do not
belong to this folio (e.g., page->_mapcount).
Sounds quite bad.
After above analysis, I think the possible race is as follows:
CPU 0 CPU 1
filemap_map_pages() ext4_setattr()
//get and lock folio with old inode->i_size
next_uptodate_folio()
.......
//shrink the inode->i_size
i_size_write(inode, attr->ia_size);
//calculate the end_pgoff with the new inode->i_size
file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
end_pgoff = min(end_pgoff, file_end);
......
//nr_pages can be overflowed, cause xas.xa_index > end_pgoff
end = folio_next_index(folio) - 1;
nr_pages = min(end, end_pgoff) - xas.xa_index + 1;
......
//map large folio
filemap_map_folio_range()
......
//truncate folios
truncate_pagecache(inode, inode->i_size);
To fix this issue, move the 'end_pgoff' calculation before next_uptodate_folio(),
so the retrieved folio stays consistent with the file end to avoid 'nr_pages'
calculation overflow. After this patch, the crash issue is gone.
Fixes: 743a2753a02e ("filemap: cap PTE range to be created to allowed zero fill in folio_map_range()")
This certainly sounds like stable material :)
Yes, will cc stable.
Reported-by: Yuanhe Shu <xiangzao@xxxxxxxxxxxxxxxxx>
Tested-by: Yuanhe Shu <xiangzao@xxxxxxxxxxxxxxxxx>
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c
index bc6775084744..923d28e59642 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3879,14 +3879,14 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
unsigned int nr_pages = 0, folio_type;
unsigned short mmap_miss = 0, mmap_miss_saved;
+ file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
+ end_pgoff = min(end_pgoff, file_end);
+
rcu_read_lock();
folio = next_uptodate_folio(&xas, mapping, end_pgoff);
if (!folio)
goto out;
- file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
- end_pgoff = min(end_pgoff, file_end);
-
/*
* Do not allow to map with PMD across i_size to preserve
* SIGBUS semantics.
LGTM. I do wonder if we want to add a comment above the file_end,
stating that this really must happen before the next_uptodate_folio()
to handle concurrent truncation.
Ack. How about adding the following comments?
"
Recalculate end_pgoff based on file_end before calling next_uptodate_folio() to avoid races with concurrent truncation.
"