Re: [PATCH] update ctime and mtime for mmaped write

[Peter Staubach <staubach@redhat.com>]

Miklos Szeredi wrote:
>>>>>>> +int set_page_dirty_mapping(struct page *page);
>>>>>>>   
>>>>>>>       
>>>>>>>           
>>>>>>>               
>>>>>> This aspect of the design seems intrusive to me.  I didn't see a strong
>>>>>> reason to introduce new versions of many of the routines just to handle
>>>>>> these semantics.  What motivated this part of your design?  Why the new
>>>>>> _mapping versions of routines?
>>>>>>     
>>>>>>         
>>>>>>             
>>>>> Because there's no way to know inside the set_page_dirty() functions
>>>>> if the dirtying comes from a memory mapping or from a modification
>>>>> through a normal write().  And they have different semantics, for
>>>>> write() the modification times are updated immediately.
>>>>>       
>>>>>           
>>>> Perhaps I didn't understand what page_mapped() does, but it does seem to
>>>> have the right semantics as far as I could see.
>>>>     
>>>>         
>>> The problems will start, when you have a file that is both mapped and
>>> modified with write().  Then the dirying from the write() will set the
>>> flag, and that will have undesirable consequences.
>>>       
>> I don't think that I quite follow the logic.  The dirtying from write()
>> will set the flag, but then the mtime will get updated and the flag will
>> be cleared by the hook in file_update_time().  Right?
>>     
>
> Take this example:
>
>     fd = open()
>     addr = mmap(.., fd)
>     write(fd, ...)
>     close(fd)
>     sleep(100)
>     msync(addr,...)
>     munmap(addr)
>
> The file times will be updated in write(), but with your patch, the
> bit in the mapping will also be set.
>
> Then in msync() the file times will be updated again, which is wrong,
> since the memory was _not_ modified through the mapping.

This is correct.  I have updated my proposed patch to include the clearing
of AS_MCTIME in the routine which updates the mtime field.  I haven't
reposted it yet until I complete testing of the new resulting system.  I
anticipate doing this later today.

    Thanx..

       ps