linux-kernel - Re: [PATCH] Use kzfree in tty buffer management to enforce data sanitization

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Date:	Sun, 31 May 2009 10:05:36 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Pekka Enberg <penberg@...helsinki.fi>,
	"Larry H." <research@...reption.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] Use kzfree in tty buffer management to enforce data 
 sanitization

On Sun, 31 May 2009, Alan Cox wrote:

> > >        memset(buf->data, 0, N_TTY_BUF_SIZE);
> > >        if (PAGE_SIZE != N_TTY_BUF_SIZE)
> > >                kfree(...)
> > >        else
> > >                free_page(...)
> > >
> > >
> > > but quite frankly, I'm not convinced about these patches at all.
> > 
> > I wonder why the tty code has that N_TTY_BUF_SIZE special casing in
> > the first place? I think we can probably just get rid of it and thus
> > we can use kzfree() here if we want to.
> 
> Some platforms with very large page sizes override the use of page based
> allocators (eg older ARM would go around allocating 32K). The normal path
> is 4K or 8K page sized buffers.

I think Pekka meant the other way around - why don't we always just use 
kmalloc(N_TTY_BUF_SIZE)/kfree(), and drop the whole conditional "use page 
allocator" entirely?

I suspect the "use page allocator" is historical - ie the tty layer 
originally always did that, and then when people wanted to suppotr smaller 
areas than one page, they added the special case. I have this dim memory 
of the _original_ kmalloc not handling page-sized allocations well (due to 
embedded size/pointer overheads), but I think all current allocators are 
perfectly happy to allocate PAGE_SIZE buffers without slop.

If I'm right, then we could just use kmalloc/kfree unconditionally. Pekka?

			Linus
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