linux-kernel - Re: [PATCH] osd fs: __r4w_get_page rely on PageUptodate for uptodate

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Date:	Mon, 02 Nov 2015 13:05:32 +0200
From:	Boaz Harrosh <ooo@...ctrozaur.com>
To:	Hugh Dickins <hughd@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Christoph Lameter <cl@...ux.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-mm@...ck.org, osd-dev@...n-osd.org
Subject: Re: [PATCH] osd fs: __r4w_get_page rely on PageUptodate for uptodate

On 11/02/2015 01:39 AM, Hugh Dickins wrote:
<>
>> This patch is not correct!
> 
> I think you have actually confirmed that the patch is correct:
> why bother to test PageDirty or PageWriteback when PageUptodate
> already tells you what you need?
> 
> Or do these filesystems do something unusual with PageUptodate
> when PageDirty is set?  I didn't find it.
> 

This is kind of delicate stuff. It took me a while to get it right
when I did it. I don't remember all the details.

But consider this option:

exofs_write_begin on a full PAGE_CACHE_SIZE, the page is instantiated
new in page-cache is that PageUptodate(page) then? I thought not.
(exofs does not set that)

Now that page I do not want to read in. The latest data is in memory.
(Same when this page is in writeback, dirty-bit is cleared)

So for sure if page is dirty or writeback then we surly do not need a read.
only if not then we need to consider the  PageUptodate(page) state.

Do you think the code is actually wrong as is?

BTW: Very similar code is in fs/nfs/objlayout/objio_osd.c::__r4w_get_page

> Thanks,
> Hugh
> 
<>

Thanks
Boaz

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