linux-ext4 - Re: [patch v2 1/5] mm: add nofail variants of kmalloc kcalloc and kzalloc

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Message-ID: <20100902145141.GA3273@quack.suse.cz>
Date:	Thu, 2 Sep 2010 16:51:41 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Neil Brown <neilb@...e.de>, Alasdair G Kergon <agk@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	Steven Whitehouse <swhiteho@...hat.com>,
	Jens Axboe <jaxboe@...ionio.com>, Jan Kara <jack@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-raid@...r.kernel.org, linux-btrfs@...r.kernel.org,
	cluster-devel@...hat.com, linux-ext4@...r.kernel.org,
	reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v2 1/5] mm: add nofail variants of kmalloc kcalloc and
 kzalloc

On Thu 02-09-10 09:59:13, Jiri Slaby wrote:
> On 09/02/2010 03:02 AM, David Rientjes wrote:
> > --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -334,6 +334,57
> > @@ static inline void *kzalloc_node(size_t size, gfp_t flags, int node)
> > return kmalloc_node(size, flags | __GFP_ZERO, node); }
> >  
> > +/** + * kmalloc_nofail - infinitely loop until kmalloc() succeeds.  +
> > * @size: how many bytes of memory are required.  + * @flags: the type
> > of memory to allocate (see kmalloc).  + * + * NOTE: no new callers of
> > this function should be implemented!  + * All memory allocations should
> > be failable whenever possible.  + */ +static inline void
> > *kmalloc_nofail(size_t size, gfp_t flags) +{ +	void *ret; + +	for
> > (;;) { +		ret = kmalloc(size, flags); +		if (ret) +
> > return ret; +		WARN_ON_ONCE(get_order(size) >
> > PAGE_ALLOC_COSTLY_ORDER);
> 
> This doesn't work as you expect. kmalloc will warn every time it fails.
> __GFP_NOFAIL used to disable the warning. Actually what's wrong with
> __GFP_NOFAIL? I cannot find a reason in the changelogs why the patches
> are needed.
  David should probably add the reasoning to the changelogs so that he
doesn't have to explain again and again ;). But if I understood it
correctly, the concern is that the looping checks slightly impact fast path
of the callers which do not need it. Generally, also looping for a long
time inside allocator isn't a nice thing but some callers aren't able to do
better for now to the patch is imperfect in this sence...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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