linux-kernel - Re: [patch 1/4] fs, jfs: remove slab object constructor

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Date:	Thu, 26 Mar 2015 08:28:00 +0100
From:	Christoph Hellwig <hch@....de>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Kleikamp <shaggy@...nel.org>,
	Christoph Hellwig <hch@....de>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	jfs-discussion@...ts.sourceforge.net
Subject: Re: [patch 1/4] fs, jfs: remove slab object constructor

On Wed, Mar 25, 2015 at 07:37:40PM -0700, David Rientjes wrote:
> That would be true only for
> 
> 	ptr = mempool_alloc(gfp, pool);
> 	mempool_free(ptr, pool);
> 
> and nothing in between, and that's pretty pointless.  Typically, callers 
> allocate memory, modify it, and then free it.  When that happens with 
> mempools, and we can't allocate slab because of the gfp context, mempools 
> will return elements in the state in which they were freed (modified, not 
> as constructed).

The historic slab allocator (Solaris and early Linux) expects objects
to be returned in the same / similar enough form as the constructor
returned it, and the constructor is only called when allocating pages
from the page pool.

I have to admit that I haven't used this feature forever, and I have no idea if
people changed how the allocator works in the meantime.
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