linux-kernel - Has slab ctor operation changed? -- was [PATCH 1/1] afs: afs_alloc_inode: use kmem_cache

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Date:	Thu, 20 Feb 2014 22:23:15 +0000
From:	David Howells <dhowells@...hat.com>
To:	Fabian Frederick <fabf@...net.be>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	akpm <akpm@...ux-foundation.org>, cl@...ux-foundation.org
Subject: Has slab ctor operation changed? -- was [PATCH 1/1] afs: afs_alloc_inode: use kmem_cache_zalloc

Fabian Frederick <fabf@...net.be> wrote:

> afs_vnode is currently cleared with 2 memsets after allocation and 
> 1 in constructor (afs_i_init_once).
> 	-This patch calls zalloc for explicit zero fill.

Ummm...  This patch isn't necessarily correct in the substantiative portions.

Since afs_i_init_once() is called by the slab allocator during the course of
kmem_cache_alloc(), how does kmem_cache_zalloc() interact with that?

IIRC, it used to be that the ctor() function was called when the pages were
allocated to the slab - and it wasn't called again, even if the object was
allocated, deallocated and reallocated.  This means that things like locks and
lists don't need reinitialising after allocation.

So afs_i_init_once() theoretically constructs the stuff that can be reused,
and afs_alloc_inode() therefore has to clear the non-reusable state.

Of course, it's possible that the slab allocator no longer works like this...

David
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