Re: What is the purpose of EXTENT_PAGE_MAPPED

[Goldwyn Rodrigues <rgoldwyn@suse.de>]

On 10/30/2017 09:01 AM, Nikolay Borisov wrote:
> 
> 
> On 30.10.2017 15:55, David Sterba wrote:
>> On Mon, Oct 30, 2017 at 03:21:51PM +0200, Nikolay Borisov wrote:
>>>
>>>
>>> On 27.10.2017 07:17, Liu Bo wrote:
>>>> On Tue, Oct 24, 2017 at 05:47:11AM -0500, Goldwyn Rodrigues wrote:
>>>>>
>>>>> EXTENT_PAGE_MAPPED gets set in set_page_extent_mapped(), but I don't see
>>>>> it being cross checked anytime. What is the purpose of setting it?
>>>>
>>>> Please check commit d1310b2e0cd98eb1348553e69b73827b436dca7b, it was
>>>> used to differentiate page for metadata and for data, but I think
>>>> currently it's just a piece of legacy code.
>>>
>>> Be that as it may - is there any reason why we are keeping this and can
>>> it be killed off?
>>
>> Are we're talking about EXTENT_PAGE_PRIVATE? There's no
>> EXTENT_PAGE_MAPPED. There's some control dependency on the page private
>> bit and the value, so we should be careful and replace the function with
>> an assert (or a BUG_ON if it's a must-not-happen state). The page->private
>> points to an extent buffer, and if it's always an eb, then the
>> EXTENT_PAGE_PRIVATE is unused.
> 
> I guess I meant do we actually need: set_page_extent_mapped and all the
> jazz happening in it  or is it a leftover (which I believe it is) ?
> 

We definitely need set_page_extent_mapped() to set the page private for
the lowers layers to handle I/O, primarily writebacks. However, we may
not need EXTENT_PAGE_PRIVATE (yes, I got it wrong the first time). I am
still reading on this though.


-- 
Goldwyn