Message-ID: <56387D3C.5020003@plexistor.com>
Date:	Tue, 03 Nov 2015 11:24:12 +0200
From:	Boaz Harrosh <boaz@...xistor.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 11/03/2015 04:49 AM, Hugh Dickins wrote:
> On Mon, 2 Nov 2015, Boaz Harrosh wrote:
>> 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:
> 
> Thanks, yes, it helps to have a concrete example in front of us.
> 
>>
>> 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.
> 
> Right, PageUptodate must not be set until the page has been filled with
> the correct data.  Nor is PageDirty or PageWriteback set at this point,
> actually.
> 
> Once page is filled with the correct data, either exofs_write_end()
> (which uses simple_write_end()) or (internally) exofs_commit_chunk()
> is called.
> 
>> (exofs does not set that)
> 
> It's simple_write_end() or exofs_commit_chunk() which SetPageUptodate
> in this case.  And after that each calls set_page_dirty(), which does
> the SetPageDirty, before unlocking the page which was supplied locked
> by exofs_write_begin().
> 
> So I don't see where the page is PageDirty without being PageUptodate.
> 
>>
>> 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)
> 
> Understood, but that's what PageUptodate is for.
> 
> (Quite what happens if there's a write error is not so clear: I think
> that typically PageError gets set and PageUptodate cleared, to read
> back in from disk what's actually there - but lose the data we wanted
> to write; but I can understand different filesystems making different
> choices there, and didn't study exofs's choice.)
> 
>>
>> 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.
> 
> PageUptodate is the proper flag to check, to ask if the page contains
> the correct data: there is no need to consider Dirty or Writeback.
> 
>>
>> Do you think the code is actually wrong as is?
> 
> Not that I know of: just a little too complicated and confusing.
> 
> But becomes slightly wrong if my simplification to page migration
> goes through, since that introduces an instant when PageDirty is set
> before the new page contains the correct data and is marked Uptodate.
> Hence my patch.
> 
>>
>> BTW: Very similar code is in fs/nfs/objlayout/objio_osd.c::__r4w_get_page
> 
> Indeed, the patch makes the same adjustment to that code too.
> 

OK thanks. Let me setup and test your patch. On top of 4.3 is good?
I'll send you a tested-by once I'm done.

Boaz

>>
>>> Thanks,
>>> Hugh
>>>
>> <>
>>
>> Thanks
>> Boaz
> 

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