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SubjectRe: [PATCH 11/13] SCSI: support for allocating large scatterlists
On Thu, 10 May 2007 12:21:53 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:

> This is what enables large commands. If we need to allocate an
> sgtable that doesn't fit in a single page, allocate several
> SCSI_MAX_SG_SEGMENTS sized tables and chain them together.
>
> We default to the safe setup of NOT chaining, for now.
>
> ...
>
> +/*
> + * Should fit within a single page, and must be a power-of-2.
> + */
> +#define SCSI_MAX_SG_SEGMENTS 128

But what units is it in? Bytes? sizeof(void*)?

> +struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
> +{
> + struct scsi_host_sg_pool *sgp;
> + struct scatterlist *sgl, *prev, *ret;
> + unsigned int index;
> + int this, left;
> +
> + BUG_ON(!cmd->use_sg);
> +
> + left = cmd->use_sg;
> + ret = prev = NULL;
> + do {
> + this = left;
> + if (this > SCSI_MAX_SG_SEGMENTS) {
> + this = SCSI_MAX_SG_SEGMENTS;
> + index = SG_MEMPOOL_NR - 1;
> + } else
> + index = scsi_sgtable_index(this);
> +
> + left -= this;
> +
> + /*
> + * if we have more entries after this round, reserve a slot
> + * for the chain pointer.
> + */
> + if (left)
> + left++;
> +
> + sgp = scsi_sg_pools + index;
> +
> + sgl = mempool_alloc(sgp->pool, gfp_mask);
> + if (unlikely(!sgl))
> + goto enomem;
> +
> + memset(sgl, 0, sizeof(*sgl) * sgp->size);
> +
> + /*
> + * first loop through, set initial index and return value
> + */
> + if (!ret) {
> + cmd->sglist_len = index;
> + ret = sgl;
> + }
> +
> + /*
> + * chain previous sglist, if any. we know the previous
> + * sglist must be the biggest one, or we would not have
> + * ended up doing another loop.
> + */
> + if (prev)
> + sg_chain(prev, SCSI_MAX_SG_SEGMENTS, sgl);
> +
> + /*
> + * don't allow subsequent mempool allocs to sleep, it would
> + * violate the mempool principle.
> + */
> + gfp_mask &= ~__GFP_WAIT;

hrm.

Might want to set __GFP_HIGH here too.

> + prev = sgl;
> + } while (left);
> +
> + /*
> + * ->use_sg may get modified after dma mapping has potentially
> + * shrunk the number of segments, so keep a copy of it for free.
> + */
> + cmd->__use_sg = cmd->use_sg;
> + return ret;
> +enomem:
> + if (ret) {
> + /*
> + * Free entries chained off ret. Since we were trying to
> + * allocate another sglist, we know that all entries are of
> + * the max size.
> + */
> + sgp = scsi_sg_pools + SG_MEMPOOL_NR - 1;
> + prev = &ret[SCSI_MAX_SG_SEGMENTS - 1];
> +
> + while ((sgl = sg_chain_ptr(ret)) != NULL) {
> + ret = &sgl[SCSI_MAX_SG_SEGMENTS - 1];
> + mempool_free(sgl, sgp->pool);
> + }
> +
> + mempool_free(prev, sgp->pool);
> + }
> + return NULL;
> }

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