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

[Peter Staubach <staubach@redhat.com>]

Miklos Szeredi wrote:
>>>>> Inspired by Peter Staubach's patch and the resulting comments.
>>>>>
>>>>>   
>>>>>       
>>>>>           
>>>> An updated version of the original patch was submitted to LKML
>>>> yesterday...  :-)
>>>>     
>>>>         
>>> Strange coincidence :)
>>>
>>>   
>>>       
>>>>>  		file = vma->vm_file;
>>>>>  		start = vma->vm_end;
>>>>> +		mapping_update_time(file);
>>>>>  		if ((flags & MS_SYNC) && file &&
>>>>>  				(vma->vm_flags & VM_SHARED)) {
>>>>>  			get_file(file);
>>>>>   
>>>>>       
>>>>>           
>>>> It seems to me that this might lead to file times being updated for
>>>> non-MAP_SHARED mappings.
>>>>     
>>>>         
>>> In theory no, because the COW-ed pages become anonymous and are not
>>> part of the original mapping any more.
>>>
>>>   
>>>       
>> I must profess to having a incomplete understanding of all of this
>> support, but then why would it be necessary to test VM_SHARED at
>> this point in msync()?
>>     
>
> That's basically just an optimization.  If it wasn't there, then data
> from a another (shared) mapping could be written back, which is not
> wrong, but not required either.
>
>   
>> I ran into problems early on with file times being updated incorrectly
>> so I am a little sensitive this aspect.
>>
>>     
>>>>> +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?

    Thanx...

       ps