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

Ingo Molnar
2009-05-09T09:20:07Z

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

> Hi Andrew,
>
> Andrew Morton wrote:
>> On Fri, 08 May 2009 18:10:28 +0300
>> Pekka Enberg <penberg@cs.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