linux-kernel - Re: [PATCH 1/2] mm: Introduce GFP_PANIC for early-boot allocations

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Date:	Sat, 9 May 2009 11:19:11 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, gorcunov@...nvz.org,
	kosaki.motohiro@...fujitsu.com, mel@....ul.ie,
	cl@...ux-foundation.org, riel@...hat.com,
	linux-kernel@...r.kernel.org, rientjes@...gle.com
Subject: Re: [PATCH 1/2] mm: Introduce GFP_PANIC for early-boot allocations


* Pekka Enberg <penberg@...helsinki.fi> wrote:

> Hi Andrew,
>
> Andrew Morton wrote:
>> On Fri, 08 May 2009 18:10:28 +0300
>> Pekka Enberg <penberg@...helsinki.fi> wrote:
>>
>>> +#define GFP_PANIC	(__GFP_NOFAIL | __GFP_NORETRY)
>>
>> urgh, you have to be kidding me.  This significantly worsens complexity
>> and risk in core MM and it's just yuk.
>>
>> I think we can justify pulling such dopey party tricks to save 
>> pageframe space, or bits in page.flags and such.  But just to 
>> save a scrap of memory which would have been released during boot 
>> anwyay?  Don't think so.
>
> No, I wasn't kidding and I don't agree that it "significantly 
> worsens complexity". The point is not to save memory but to 
> clearly annotate those special call-sites that really don't need 
> to check for out-of-memory.

Frankly, i cannot believe that so many smart people dont see the 
simple, universal, un-arguable truism in the following statement:

 it is shorter, tidier, more maintainable, more reviewable to write:

	ptr = kmalloc(GFP_BOOT, size);

 than to write:

	ptr = kmalloc(GFP_KERNEL, size);
	BUG_ON(!ptr);

We have a lot of such patterns in platform code. Dozens and dozens 
of them.

There _might_ be some more nuanced second-level arguments: "yes, I 
agree in principle, but complexity elsewhere or other side-effects 
make this a net negative change."

Alas, those arguments would be wrong too:

 - we have a lot of such patterns and GFP_BOOT is unambigious 

 - post-bootup mis-use of GFP_BOOT could be warned about 
   unconditionally if used after free_initmem(), if we care enough.

 - Pekka's patch is dead simple. There's no "complexity" anywhere.

Agreeing to this and introducing this should have been a matter of 
30 seconds of thinking. Why are we still arguing about this? Dont we 
have enough bugs to worry about?

	Ingo
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