[PATCH 4/9] btrfs: Avoid a rescan for a device which was already not found.

From: Goffredo Baroncelli <kreijack@libero.it>
To: grub-devel@gnu.org
Cc: Daniel Kiper <dkiper@net-space.pl>,
	linux-btrfs@vger.kernel.org,
	Goffredo Baroncelli <kreijack@inwind.it>
Subject: [PATCH 4/9] btrfs: Avoid a rescan for a device which was already not found.
Date: Thu, 27 Sep 2018 20:34:59 +0200	[thread overview]
Message-ID: <20180927183504.18078-5-kreijack@libero.it> (raw)
In-Reply-To: <20180927183504.18078-1-kreijack@libero.it>

From: Goffredo Baroncelli <kreijack@inwind.it>

Change the behavior of find_device(): before the patch, a read of a missed
device may trigger a rescan. However, it is never recorded that a device
is missed, so each single read of a missed device may triggers a rescan.
It is the caller who decides if a rescan is performed in case of a missed
device. And it does quite often, without considering if in the past a
devices was already found as "missed"
This behavior causes a lot of unneeded rescan, causing a huge slowdown in
case of a missed device.

After the patch, the "missed device" information is stored in the
data->devices_attached[] array (as a NULL value in the field dev). A rescan
is triggered only if no information at all is found. This means that only
the first time a read of a missed device triggers a rescan.

The change in the code is done removing "return NULL" when the disk is not
found. So it is always executed the code which stores in the
data->devices_attached[] array the value returned by grub_device_iterate():
NULL if the device is missed, or a valid data otherwise.

Signed-off-by: Goffredo Baroncelli <kreikack@inwind.it>
---
 grub-core/fs/btrfs.c | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/grub-core/fs/btrfs.c b/grub-core/fs/btrfs.c
index 2b1dd7dd4..c07d91657 100644
--- a/grub-core/fs/btrfs.c
+++ b/grub-core/fs/btrfs.c
@@ -588,7 +588,7 @@ find_device_iter (const char *name, void *data)
 }
 
 static grub_device_t
-find_device (struct grub_btrfs_data *data, grub_uint64_t id, int do_rescan)
+find_device (struct grub_btrfs_data *data, grub_uint64_t id)
 {
   struct find_device_ctx ctx = {
     .data = data,
@@ -600,10 +600,9 @@ find_device (struct grub_btrfs_data *data, grub_uint64_t id, int do_rescan)
   for (i = 0; i < data->n_devices_attached; i++)
     if (id == data->devices_attached[i].id)
       return data->devices_attached[i].dev;
-  if (do_rescan)
-    grub_device_iterate (find_device_iter, &ctx);
-  if (!ctx.dev_found)
-    return NULL;
+
+  grub_device_iterate (find_device_iter, &ctx);
+
   data->n_devices_attached++;
   if (data->n_devices_attached > data->n_devices_allocated)
     {
@@ -615,7 +614,8 @@ find_device (struct grub_btrfs_data *data, grub_uint64_t id, int do_rescan)
 			* sizeof (data->devices_attached[0]));
       if (!data->devices_attached)
 	{
-	  grub_device_close (ctx.dev_found);
+	  if (ctx.dev_found)
+	    grub_device_close (ctx.dev_found);
 	  data->devices_attached = tmp;
 	  return NULL;
 	}
@@ -898,7 +898,7 @@ grub_btrfs_read_logical (struct grub_btrfs_data *data, grub_disk_addr_t addr,
 			      " for laddr 0x%" PRIxGRUB_UINT64_T "\n", paddr,
 			      addr);
 
-		dev = find_device (data, stripe->device_id, j);
+		dev = find_device (data, stripe->device_id);
 		if (!dev)
 		  {
 		    grub_dprintf ("btrfs",
@@ -975,7 +975,8 @@ grub_btrfs_unmount (struct grub_btrfs_data *data)
   unsigned i;
   /* The device 0 is closed one layer upper.  */
   for (i = 1; i < data->n_devices_attached; i++)
-    grub_device_close (data->devices_attached[i].dev);
+    if (data->devices_attached[i].dev)
+        grub_device_close (data->devices_attached[i].dev);
   grub_free (data->devices_attached);
   grub_free (data->extent);
   grub_free (data);
-- 
2.19.0

