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Subject[PATCH 12/14] scatterlist: allow chaining to preallocated chunks
Date
Blk-mq drivers usually preallocate their S/G list as part of the request,
but if we want to support the very large S/G lists currently supported by
the SCSI code that would tie up a lot of memory in the preallocated request
pool. Add support to the scatterlist code so that it can initialize a
S/G list that uses a preallocated first chunks and dynamically allocated
additional chunks. That way the scsi-mq code can preallocate a first
page worth of S/G entries as part of the request, and dynamically extent
the S/G list when needed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
drivers/scsi/scsi_lib.c | 16 +++++++---------
include/linux/scatterlist.h | 6 +++---
lib/scatterlist.c | 24 ++++++++++++++++--------
3 files changed, 26 insertions(+), 20 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index e438726..32fbae4 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -567,6 +567,11 @@ static struct scatterlist *scsi_sg_alloc(unsigned int nents, gfp_t gfp_mask)
return mempool_alloc(sgp->pool, gfp_mask);
}

+static void scsi_free_sgtable(struct scsi_data_buffer *sdb)
+{
+ __sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS, false, scsi_sg_free);
+}
+
static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents,
gfp_t gfp_mask)
{
@@ -575,19 +580,12 @@ static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents,
BUG_ON(!nents);

ret = __sg_alloc_table(&sdb->table, nents, SCSI_MAX_SG_SEGMENTS,
- gfp_mask, scsi_sg_alloc);
+ NULL, gfp_mask, scsi_sg_alloc);
if (unlikely(ret))
- __sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS,
- scsi_sg_free);
-
+ scsi_free_sgtable(sdb);
return ret;
}

-static void scsi_free_sgtable(struct scsi_data_buffer *sdb)
-{
- __sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS, scsi_sg_free);
-}
-
/*
* Function: scsi_release_buffers()
*
diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index a964f72..f4ec8bb 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -229,10 +229,10 @@ void sg_init_one(struct scatterlist *, const void *, unsigned int);
typedef struct scatterlist *(sg_alloc_fn)(unsigned int, gfp_t);
typedef void (sg_free_fn)(struct scatterlist *, unsigned int);

-void __sg_free_table(struct sg_table *, unsigned int, sg_free_fn *);
+void __sg_free_table(struct sg_table *, unsigned int, bool, sg_free_fn *);
void sg_free_table(struct sg_table *);
-int __sg_alloc_table(struct sg_table *, unsigned int, unsigned int, gfp_t,
- sg_alloc_fn *);
+int __sg_alloc_table(struct sg_table *, unsigned int, unsigned int,
+ struct scatterlist *, gfp_t, sg_alloc_fn *);
int sg_alloc_table(struct sg_table *, unsigned int, gfp_t);
int sg_alloc_table_from_pages(struct sg_table *sgt,
struct page **pages, unsigned int n_pages,
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 3a8e8e8..48c15d2 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -165,6 +165,7 @@ static void sg_kfree(struct scatterlist *sg, unsigned int nents)
* __sg_free_table - Free a previously mapped sg table
* @table: The sg table header to use
* @max_ents: The maximum number of entries per single scatterlist
+ * @skip_first_chunk: don't free the (preallocated) first scatterlist chunk
* @free_fn: Free function
*
* Description:
@@ -174,7 +175,7 @@ static void sg_kfree(struct scatterlist *sg, unsigned int nents)
*
**/
void __sg_free_table(struct sg_table *table, unsigned int max_ents,
- sg_free_fn *free_fn)
+ bool skip_first_chunk, sg_free_fn *free_fn)
{
struct scatterlist *sgl, *next;

@@ -202,7 +203,9 @@ void __sg_free_table(struct sg_table *table, unsigned int max_ents,
}

table->orig_nents -= sg_size;
- free_fn(sgl, alloc_size);
+ if (!skip_first_chunk)
+ free_fn(sgl, alloc_size);
+ skip_first_chunk = false;
sgl = next;
}

@@ -217,7 +220,7 @@ EXPORT_SYMBOL(__sg_free_table);
**/
void sg_free_table(struct sg_table *table)
{
- __sg_free_table(table, SG_MAX_SINGLE_ALLOC, sg_kfree);
+ __sg_free_table(table, SG_MAX_SINGLE_ALLOC, false, sg_kfree);
}
EXPORT_SYMBOL(sg_free_table);

@@ -241,8 +244,8 @@ EXPORT_SYMBOL(sg_free_table);
*
**/
int __sg_alloc_table(struct sg_table *table, unsigned int nents,
- unsigned int max_ents, gfp_t gfp_mask,
- sg_alloc_fn *alloc_fn)
+ unsigned int max_ents, struct scatterlist *first_chunk,
+ gfp_t gfp_mask, sg_alloc_fn *alloc_fn)
{
struct scatterlist *sg, *prv;
unsigned int left;
@@ -269,7 +272,12 @@ int __sg_alloc_table(struct sg_table *table, unsigned int nents,

left -= sg_size;

- sg = alloc_fn(alloc_size, gfp_mask);
+ if (first_chunk) {
+ sg = first_chunk;
+ first_chunk = NULL;
+ } else {
+ sg = alloc_fn(alloc_size, gfp_mask);
+ }
if (unlikely(!sg)) {
/*
* Adjust entry count to reflect that the last
@@ -324,9 +332,9 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
int ret;

ret = __sg_alloc_table(table, nents, SG_MAX_SINGLE_ALLOC,
- gfp_mask, sg_kmalloc);
+ NULL, gfp_mask, sg_kmalloc);
if (unlikely(ret))
- __sg_free_table(table, SG_MAX_SINGLE_ALLOC, sg_kfree);
+ __sg_free_table(table, SG_MAX_SINGLE_ALLOC, false, sg_kfree);

return ret;
}
--
1.7.10.4


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