[PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()

[PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()

[Baolin Wang]

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@linux.alibaba.com>
Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
Changes from RFC:
 - Add acked tag from Kiryl. Thanks.
 - Add some comments and CC stable, per David.
---
 mm/filemap.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index bc6775084744..598890871635 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3879,14 +3879,19 @@ 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;
 
+	/*
+	 * Recalculate end_pgoff based on file_end before calling
+	 * next_uptodate_folio() to avoid races with concurrent
+	 * truncation.
+	 */
+	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.
-- 
2.47.3

Re: [PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()

[David Hildenbrand (Arm)]

On 3/17/26 10:29, 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.
> 

Thanks!

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David

Re: [PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()

[David Hildenbrand (Arm)]

On 3/17/26 10:37, David Hildenbrand (Arm) wrote:
> On 3/17/26 10:29, 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.
>>
> 
> Thanks!
> 
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> 

I just skimmed over the AI review:

https://sashiko.dev/#/patchset/1cf1ac59018fc647a87b0dad605d4056a71c14e4.1773739704.git.baolin.wang%40linux.alibaba.com

And I'm not sure if it has a point, in particular whether
i_size_read(mapping->host) could return 0 and underflow file_end.

I'd assume, in that case (truncation succeeded), also the
next_uptodate_folio() would fail.

But it's tricky :)

-- 
Cheers,

David

Re: [PATCH] mm: filemap: fix nr_pages calculation overflow in filemap_map_pages()

[Baolin Wang]

On 3/18/26 4:49 AM, David Hildenbrand (Arm) wrote:
> On 3/17/26 10:37, David Hildenbrand (Arm) wrote:
>> On 3/17/26 10:29, 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.
>>>
>>
>> Thanks!
>>
>> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks for reviewing.

> I just skimmed over the AI review:
> 
> https://sashiko.dev/#/patchset/1cf1ac59018fc647a87b0dad605d4056a71c14e4.1773739704.git.baolin.wang%40linux.alibaba.com

Thanks. Zi Yan also sent me the AI-generated comments, and I don’t think 
this is an issue.

> And I'm not sure if it has a point, in particular whether
> i_size_read(mapping->host) could return 0 and underflow file_end.
> 
> I'd assume, in that case (truncation succeeded), also the
> next_uptodate_folio() would fail.

Yes. If truncation has succeeded, next_uptodate_folio() cannot find a 
folio. If called before truncation, next_uptodate_folio() also checks 
i_size_read(mapping->host) and returns NULL:

max_idx = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE);
if (xas->xa_index >= max_idx)
	goto unlock;

So I don't think this will cause a real issue if 
i_size_read(mapping->host) returns 0.