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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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