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@xxxxxxxxxx>

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.