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Subject[COMPILE ONLY RFC PATCH] loop: avoid page allocation in the fast path
Date
The functions lo_write_transfer() and lo_read_transfer() allocte and
free the page everytime it is called from the do_req_filebacked().

Add a member to the loop_cmd structure and use that page in read/write
transfer functions.

This is *compile only* RFC, if this is accepted I'll send tested
version.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
---
drivers/block/loop.c | 22 ++++++++--------------
drivers/block/loop.h | 1 +
2 files changed, 9 insertions(+), 14 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 578fc034db3f..08bd17afbdff 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -318,14 +318,13 @@ static int lo_write_simple(struct loop_device *lo, struct request *rq,
* access to the destination pages of the backing file.
*/
static int lo_write_transfer(struct loop_device *lo, struct request *rq,
- loff_t pos)
+ struct loop_cmd *cmd, loff_t pos)
{
- struct bio_vec bvec, b;
+ struct page *page = &cmd->page;
struct req_iterator iter;
- struct page *page;
+ struct bio_vec bvec, b;
int ret = 0;

- page = alloc_page(GFP_NOIO);
if (unlikely(!page))
return -ENOMEM;

@@ -343,7 +342,6 @@ static int lo_write_transfer(struct loop_device *lo, struct request *rq,
break;
}

- __free_page(page);
return ret;
}

@@ -377,19 +375,15 @@ static int lo_read_simple(struct loop_device *lo, struct request *rq,
}

static int lo_read_transfer(struct loop_device *lo, struct request *rq,
- loff_t pos)
+ struct loop_cmd *cmd, loff_t pos)
{
- struct bio_vec bvec, b;
+ struct page *page = &cmd->page;
struct req_iterator iter;
+ struct bio_vec bvec, b;
struct iov_iter i;
- struct page *page;
ssize_t len;
int ret = 0;

- page = alloc_page(GFP_NOIO);
- if (unlikely(!page))
- return -ENOMEM;
-
rq_for_each_segment(bvec, rq, iter) {
loff_t offset = pos;

@@ -622,14 +616,14 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
return lo_fallocate(lo, rq, pos, FALLOC_FL_PUNCH_HOLE);
case REQ_OP_WRITE:
if (lo->transfer)
- return lo_write_transfer(lo, rq, pos);
+ return lo_write_transfer(lo, rq, cmd, pos);
else if (cmd->use_aio)
return lo_rw_aio(lo, cmd, pos, WRITE);
else
return lo_write_simple(lo, rq, pos);
case REQ_OP_READ:
if (lo->transfer)
- return lo_read_transfer(lo, rq, pos);
+ return lo_read_transfer(lo, rq, cmd, pos);
else if (cmd->use_aio)
return lo_rw_aio(lo, cmd, pos, READ);
else
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index a3c04f310672..dd949b22e9c9 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -71,6 +71,7 @@ struct loop_cmd {
atomic_t ref; /* only for aio */
long ret;
struct kiocb iocb;
+ struct page page;
struct bio_vec *bvec;
struct cgroup_subsys_state *css;
};
--
2.22.1
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