Re: [f2fs-dev] [PATCH] f2fs: disallow mmap write and data-modifying fallocate on atomic files

[Chao Yu <chao@kernel.org>]

On 10/9/26 23:05, Daeho Jeong wrote:
> On Fri, Oct 9, 2026 at 1:35 AM Chao Yu <chao@kernel.org> wrote:
>>
>> On 10/3/26 01:17, Daeho Jeong wrote:
>>> From: Daeho Jeong <daehojeong@google.com>
>>>
>>> Stores through a shared writable mapping and fallocate with
>>> FALLOC_FL_PUNCH_HOLE, FALLOC_FL_ZERO_RANGE, FALLOC_FL_COLLAPSE_RANGE or
>>> FALLOC_FL_INSERT_RANGE do not go through ->write_begin, so they bypass
>>> the COW inode and modify the original inode directly. Reject them while
>>> the file is in atomic write mode, as fallocate already does for pinned
>>> and compressed files. A write fault gets SIGBUS and fallocate gets
>>> -EOPNOTSUPP.
>>>
>>> fallocate without these flags only preallocates blocks and stays
>>> allowed.
>>>
>>> Fixes: 3db1de0e582c ("f2fs: change the current atomic write way")
>>> Signed-off-by: Daeho Jeong <daehojeong@google.com>
>>> ---
>>>  fs/f2fs/file.c | 10 ++++++++--
>>>  1 file changed, 8 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>> index ef4d218e694..56c686b8580 100644
>>> --- a/fs/f2fs/file.c
>>> +++ b/fs/f2fs/file.c
>>> @@ -140,6 +140,10 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
>>>               return VM_FAULT_SIGBUS;
>>>       }
>>>
>>> +     /* mmap stores bypass the COW inode and would break atomicity */
>>> +     if (f2fs_is_atomic_file(inode))
>>> +             return VM_FAULT_SIGBUS;
>>
>> Will we suffer race issue due to the check w/o lock?
>>
>> Thanks,
> 
> Hi Chao,
> 
> Yes, there is a race with f2fs_ioc_start_atomic_write().
> 
> page_mkwrite cannot take inode_lock, because the lock order is i_rwsem
> -> mmap_lock (e.g. fiemap copies to user under inode_lock). So v2 will
> use invalidate_lock instead:
> 
> f2fs_ioc_start_atomic_write() holds filemap_invalidate_lock() from the
> flush until FI_ATOMIC_FILE is set.
> f2fs_vm_page_mkwrite() rechecks the flag after
> filemap_invalidate_lock_shared(), which it holds until the folio is
> dirtied.
> 
> If you are okay with this, I'll send v2.

Daeho, looks fine to me, please go ahead. :)

Thanks,

> 
> Thanks,
> 
>>
>>> +
>>>       if (is_inode_flag_set(inode, FI_COMPRESS_RELEASED)) {
>>>               err = -EIO;
>>>               goto out;
>>> @@ -2113,9 +2117,11 @@ static long f2fs_fallocate(struct file *file, int mode,
>>>
>>>       /*
>>>        * Pinned file should not support partial truncation since the block
>>> -      * can be used by applications.
>>> +      * can be used by applications. Atomic files should not either, since
>>> +      * these modify the original inode directly and break atomicity.
>>>        */
>>> -     if ((f2fs_compressed_file(inode) || f2fs_is_pinned_file(inode)) &&
>>> +     if ((f2fs_compressed_file(inode) || f2fs_is_pinned_file(inode) ||
>>> +          f2fs_is_atomic_file(inode)) &&
>>>               (mode & (FALLOC_FL_PUNCH_HOLE | FALLOC_FL_COLLAPSE_RANGE |
>>>                       FALLOC_FL_ZERO_RANGE | FALLOC_FL_INSERT_RANGE))) {
>>>               ret = -EOPNOTSUPP;
>>