linux-kernel - Re: [PATCH][1/2]page_fault retry with NOPAGE

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Date:	Fri, 10 Apr 2009 10:57:12 -0700
From:	Ying Han <yinghan@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Waychison <mikew@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>,
	Hugh Dickins <hugh@...itas.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Török Edwin <edwintorok@...il.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][1/2]page_fault retry with NOPAGE_RETRY

2009/4/10 Linus Torvalds <torvalds@...ux-foundation.org>:
>
>
> On Fri, 10 Apr 2009, Wu Fengguang wrote:
>
>> On Fri, Apr 10, 2009 at 02:02:05PM +0800, Andrew Morton wrote:
>>
>> > Can we please redo this as:
>> >
>> >
>> >     int write;
>> >     unsigned int flags;
>> >
>> >     /*
>> >      * Big fat comment explaining the next three lines goes here
>> >      */
>>
>> Basically it's doing a
>>         (is_write_access  | FAULT_FLAG_RETRY) =>
>>         (FAULT_FLAG_WRITE | FAULT_FLAG_RETRY)
>> by extracting the bool part:
>> >     write = write_access & ~FAULT_FLAG_RETRY;
>> convert bool to a bit flag:
>> >     unsigned int flags = (write ? FAULT_FLAG_WRITE : 0);

Fengguang: thank you for your comments. i agree this is confusing...
>
> The point is, we shouldn't do that.
>
> Your code is confused, because it uses "write_access" as if it had the old
> behaviour (boolean to say "write") _plus_ the new behavior (bitmask to say
> "retry"), and that's just wrong.
>
> Just get rid of "write_access" entirely, and switch it over to something
> that is a pure bitmask.
>
> Yes, it means a couple of new preliminary patches that switch all callers
> of handle_mm_fault() over to using the VM_FLAGS, but that's not a big
> deal.
>
> I'm following up this email with two _example_ patches. They are untested,
> but they look sane. I'd like the series to _start_ with these, and then
> you can pass FAULT_FLAGS_WRITE | FAULT_FLAGS_RETRY down to
> handle_mm_fault() cleanly.
>
> Hmm? Note the _untested_ part on the patches to follow. It was done very
> mechanically, and the patches look sane, but .. !!!

Thanks Linus for your comments. I will take Peter Zijlstra's patches
(the _untested_ part)
which basically replaces the write_access as a flag as you mentioned
and start from there.

My next step is to cleanup the patch with comments in the thread so
far and post the new version.
Anything else i missed, please let me know. thanks

--Ying
>
>                        Linus
>
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