linux-kernel - Re: [Xen-devel] [PATCH] xen-blkback: use bigger array for batch gnt operations

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Date:	Thu, 1 Aug 2013 13:30:36 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Roger Pau Monne <roger.pau@...rix.com>
CC:	<xen-devel@...ts.xen.org>, <linux-kernel@...r.kernel.org>,
	David Vrabel <david.vrabel@...rix.com>
Subject: Re: [Xen-devel] [PATCH] xen-blkback: use bigger array for batch gnt
 operations

On 01/08/13 13:08, Roger Pau Monne wrote:
> Right now the maximum number of grant operations that can be batched
> in a single request is BLKIF_MAX_SEGMENTS_PER_REQUEST (11). This was
> OK before indirect descriptors because the maximum number of segments
> in a request was 11, but with the introduction of indirect
> descriptors the maximum number of segments in a request has been
> increased past 11.
> 
> The memory used by the structures that are passed in the hypercall was
> allocated from the stack, but if we have to increase the size of the
> array we can no longer use stack memory, so we have to pre-allocate
> it.
> 
> This patch increases the maximum size of batch grant operations and
> replaces the use of stack memory with pre-allocated memory, that is
> reserved when the blkback instance is initialized.
[...]
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
[...]
> @@ -148,6 +155,16 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>  			if (!req->indirect_pages[j])
>  				goto fail;
>  		}
> +		req->map = kcalloc(GNT_OPERATIONS_SIZE, sizeof(req->map[0]), GFP_KERNEL);
> +		if (!req->map)
> +			goto fail;
> +		req->unmap = kcalloc(GNT_OPERATIONS_SIZE, sizeof(req->unmap[0]), GFP_KERNEL);
> +		if (!req->unmap)
> +			goto fail;
> +		req->pages_to_gnt = kcalloc(GNT_OPERATIONS_SIZE, sizeof(req->pages_to_gnt[0]),
> +		                            GFP_KERNEL);
> +		if (!req->pages_to_gnt)
> +			goto fail;

Do these need to be per-request? Or can they all share a common set of
arrays?

David
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