linux-kernel - Re: [RFC] propagate gfp_t to page table alloc functions

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Date:	Tue, 1 May 2012 17:20:52 +1000
From:	Nick Piggin <npiggin@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Minchan Kim <minchan@...nel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [RFC] propagate gfp_t to page table alloc functions

On 25 April 2012 07:30, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Tue, 24 Apr 2012 17:48:29 +1000
> Nick Piggin <npiggin@...il.com> wrote:
>
>> > Hmm, there are several places to use GFP_NOIO and GFP_NOFS even, GFP_ATOMIC.
>> > I believe it's not trivial now.
>>
>> They're all buggy then. Unfortunately not through any real fault of their own.
>
> There are gruesome problems in block/blk-throttle.c (thread "mempool,
> percpu, blkcg: fix percpu stat allocation and remove stats_lock").  It
> wants to do an alloc_percpu()->vmalloc() from the IO submission path,
> under GFP_NOIO.

Yeah, that sucks. CFQ has something similar.

Should just allocate it up front when creating a throttled group.
Allocate and init when it first gets used schemes are usually pretty
problematic. Is it *really* warranted to do it lazily like this?

> Changing vmalloc() to take a gfp_t does make lots of sense, although I
> worry a bit about making vmalloc() easier to use!
>
> I do wonder whether the whole scheme of explicitly passing a gfp_t was
> a mistake and that the allocation context should be part of the task
> context.  ie: pass the allocation mode via *current.  As a handy
> side-effect that would probably save quite some code where functions
> are receiving a gfp_t arg then simply passing it on to the next
> callee.

Both paragraphs make a lot of sense. Conceptually. :)
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