linux-kernel - Re: [PATCH 1/5] xen-blkback: don't store dev_bus

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Message-ID: <20130319143217.GC32432@phenom.dumpdata.com>
Date:	Tue, 19 Mar 2013 10:32:17 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Roger Pau Monne <roger.pau@...rix.com>
Cc:	xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] xen-blkback: don't store dev_bus_addr

On Mon, Mar 18, 2013 at 05:49:32PM +0100, Roger Pau Monne wrote:
> dev_bus_addr returned in the grant ref map operation is the mfn of the
> passed page, there's no need to store it in the persistent grant
> entry, since we can always get it provided that we have the page.
> 
> This reduces the memory overhead of persistent grants in blkback.

I took this patch, but I redid it a bit:

commit 1d4cb410befdb8b373c6fad604b39e0200e0bee0
Author: Roger Pau Monne <roger.pau@...rix.com>
Date:   Mon Mar 18 17:49:32 2013 +0100

    xen-blkback: don't store dev_bus_addr
    
    dev_bus_addr returned in the grant ref map operation is the mfn of the
    passed page, there's no need to store it in the persistent grant
    entry, since we can always get it provided that we have the page.
    
    This reduces the memory overhead of persistent grants in blkback.
    
    While at it, rename the 'seg[i].buf' to be 'seg[i].offset' as
    it makes much more sense - as we use that value in bio_add_page
    which as the fourth argument expects the offset.
    
    We hadn't used the physical address as part of this at all.
    
    Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
    Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
    Cc: xen-devel@...ts.xen.org
    [v1: s/buf/offset/]
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index 2cf8381..061c202 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -442,7 +442,7 @@ int xen_blkif_schedule(void *arg)
 }
 
 struct seg_buf {
-	unsigned long buf;
+	unsigned long offset;
 	unsigned int nsec;
 };
 /*
@@ -621,30 +621,21 @@ static int xen_blkbk_map(struct blkif_request *req,
 				 * If this is a new persistent grant
 				 * save the handler
 				 */
-				persistent_gnts[i]->handle = map[j].handle;
-				persistent_gnts[i]->dev_bus_addr =
-					map[j++].dev_bus_addr;
+				persistent_gnts[i]->handle = map[j++].handle;
 			}
 			pending_handle(pending_req, i) =
 				persistent_gnts[i]->handle;
 
 			if (ret)
 				continue;
-
-			seg[i].buf = persistent_gnts[i]->dev_bus_addr |
-				(req->u.rw.seg[i].first_sect << 9);
 		} else {
-			pending_handle(pending_req, i) = map[j].handle;
+			pending_handle(pending_req, i) = map[j++].handle;
 			bitmap_set(pending_req->unmap_seg, i, 1);
 
-			if (ret) {
-				j++;
+			if (ret)
 				continue;
-			}
-
-			seg[i].buf = map[j++].dev_bus_addr |
-				(req->u.rw.seg[i].first_sect << 9);
 		}
+		seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
 	}
 	return ret;
 }
@@ -971,7 +962,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
 		       (bio_add_page(bio,
 				     pages[i],
 				     seg[i].nsec << 9,
-				     seg[i].buf & ~PAGE_MASK) == 0)) {
+				     seg[i].offset & ~PAGE_MASK) == 0)) {
 
 			bio = bio_alloc(GFP_KERNEL, nseg-i);
 			if (unlikely(bio == NULL))
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index da78346..60103e2 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -187,7 +187,6 @@ struct persistent_gnt {
 	struct page *page;
 	grant_ref_t gnt;
 	grant_handle_t handle;
-	uint64_t dev_bus_addr;
 	struct rb_node node;
 };
 
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