Re: [RFC PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()
From: Baolin Wang
Date: Mon Mar 16 2026 - 21:08:50 EST
On 3/16/26 8:00 PM, Kiryl Shutsemau wrote:
On Fri, Mar 13, 2026 at 01:54:31PM +0800, Baolin Wang wrote:
On 3/13/26 1:14 PM, Dev Jain wrote:
On 13/03/26 10:41 am, Dev Jain wrote:
On 13/03/26 9:15 am, 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.
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).
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()")
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.
I am wondering whether something similar can happen in the do-while loop
below this code. We can retrieve a folio from next_uptodate_folio, and
then a massive truncate happens and we end up mapping a large folio
into the pagetables beyong i_size, violating SIGBUS semantics. (truncation
may back-off seeing the locked folio/increased refcount in filemap_map_pages)
Read the bracket text as - (truncation may fail to unmap this folio seeing
it locked or with elevated refcount, therefore the illegal mapping stays
permanent)
IMHO, the truncate_pagecache() will call unmap_mapping_range() twice, and
the folio lock and refcount will not block unmap_mapping_range() to unmap
the folio's mapping (only hold ptl lock).
So the truncate_pagecache() can still truncate large folios beyond i_size.
Yeah, we serialize here on the folio lock. It should be safe.
The fix looks sane to me:
Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
Thanks for reviewing.