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

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Date:	Wed, 21 Feb 2007 13:54:57 -0500
From:	Peter Staubach <staubach@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	akpm@...ux-foundation.org, hugh@...itas.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] update ctime and mtime for mmaped write

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()?

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.

    Thanx...

       ps
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