[COMPILE ONLY RFC PATCH] loop: avoid page allocation in the fast path

[Chaitanya Kulkarni]

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.kulka...@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