linux-kernel - Re: [RFC v11][PATCH 03/13] General infrastructure for checkpoint restart

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Date:	Tue, 16 Dec 2008 13:54:28 -0800
From:	Mike Waychison <mikew@...gle.com>
To:	Oren Laadan <orenl@...columbia.edu>
CC:	jeremy@...p.org, arnd@...db.de, linux-api@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org,
	Dave Hansen <dave@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	Linux Torvalds <torvalds@...l.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC v11][PATCH 03/13] General infrastructure for checkpoint
 restart

Oren Laadan wrote:
> diff --git a/checkpoint/sys.c b/checkpoint/sys.c
> index 375129c..bd14ef9 100644
> --- a/checkpoint/sys.c
> +++ b/checkpoint/sys.c

> +/*
> + * During checkpoint and restart the code writes outs/reads in data
> + * to/from the checkpoint image from/to a temporary buffer (ctx->hbuf).
> + * Because operations can be nested, use cr_hbuf_get() to reserve space
> + * in the buffer, then cr_hbuf_put() when you no longer need that space.
> + */

This seems a bit over-kill for buffer management no?  The only large 
header seems to be cr_hdr_head and the blowup comes from utsinfo string 
data (which could easily be moved out to be in it's own CR_HDR_STRING 
blocks).

Wouldn't it be easier to use stack-local storage than balancing the 
cr_hbuf_get/put routines?

> +
> +/*
> + * ctx->hbuf is used to hold headers and data of known (or bound),
> + * static sizes. In some cases, multiple headers may be allocated in
> + * a nested manner. The size should accommodate all headers, nested
> + * or not, on all archs.
> + */
> +#define CR_HBUF_TOTAL  (8 * 4096)
> +
> +/**
> + * cr_hbuf_get - reserve space on the hbuf
> + * @ctx: checkpoint context
> + * @n: number of bytes to reserve
> + *
> + * Returns pointer to reserved space
> + */
> +void *cr_hbuf_get(struct cr_ctx *ctx, int n)
> +{
> +	void *ptr;
> +
> +	/*
> +	 * Since requests depend on logic and static header sizes (not on
> +	 * user data), space should always suffice, unless someone either
> +	 * made a structure bigger or call path deeper than expected.
> +	 */
> +	BUG_ON(ctx->hpos + n > CR_HBUF_TOTAL);
> +	ptr = ctx->hbuf + ctx->hpos;
> +	ctx->hpos += n;
> +	return ptr;
> +}
> +
> +/**
> + * cr_hbuf_put - unreserve space on the hbuf
> + * @ctx: checkpoint context
> + * @n: number of bytes to reserve
> + */
> +void cr_hbuf_put(struct cr_ctx *ctx, int n)
> +{
> +	BUG_ON(ctx->hpos < n);
> +	ctx->hpos -= n;
> +}
> +
--
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