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

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Message-Id: <20140221054752.3905832359384e0f427ee9f8@skynet.be>
Date:	Fri, 21 Feb 2014 05:47:52 +0800
From:	Fabian Frederick <fabf@...net.be>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-kernel@...r.kernel.org, akpm <akpm@...ux-foundation.org>,
	cl@...ux-foundation.org
Subject: Re: Has slab ctor operation changed? -- was [PATCH 1/1] afs:
 afs_alloc_inode: use kmem_cache_zalloc

On Thu, 20 Feb 2014 22:23:15 +0000
David Howells <dhowells@...hat.com> wrote:

> 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?

Object of this patch was to replace any kmem_cache_alloc by kmem_cache_zalloc
so I guess what is done in the patch v2 in afs_alloc_inode below zalloc
would be ok (leaving ctor how it is) ?

> 
> 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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