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

[RFC 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>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
---
 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.
-- 
2.47.3

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

[Dev Jain]

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@linux.alibaba.com>
> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  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)

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

[Dev Jain]

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@linux.alibaba.com>
>> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>  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)

> 
> 
> 
> 

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

[Baolin Wang]

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@linux.alibaba.com>
>>> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>> ---
>>>   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.

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

[Kiryl Shutsemau]

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@linux.alibaba.com>
> > > > Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
> > > > Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> > > > ---
> > > >   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@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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

[David Hildenbrand (Arm)]

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.

In particular this here:

https://syzkaller.appspot.com/bug?extid=c0673e1f1f054fac28c2

via

https://lore.kernel.org/all/6758f0cc.050a0220.17f54a.0001.GAE@google.com/

Could that be the same root cause?

> 
> 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 :)

> Reported-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  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.

-- 
Cheers,

David

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

[Baolin Wang]

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@linux.alibaba.com>
>>>>> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
>>>>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>>>>> ---
>>>>>    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@kernel.org>

Thanks for reviewing.

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

[Baolin Wang]

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@google.com/
> 
> 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@linux.alibaba.com>
>> Tested-by: Yuanhe Shu <xiangzao@linux.alibaba.com>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>>   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.
"

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

[David Hildenbrand (Arm)]

>> 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.
> "

Works for me, thanks!

-- 
Cheers,

David