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[PATCH V1] hostdisk: Fix linux disk cache workaround on multipath disks

From: Michael Chang
Subject: [PATCH V1] hostdisk: Fix linux disk cache workaround on multipath disks
Date: Thu, 13 Sep 2018 15:20:34 +0800
User-agent: NeoMutt/20170421 (1.8.2)

In grub-core/osdep/linux/hostdisk.c::grub_util_fd_open_device() there's comment
about linux disk cache issue as below:

  /* Linux has a bug that the disk cache for a whole disk is not consistent
     with the one for a partition of the disk.  */

As the input argument of grub_util_fd_open_device() is using address in unit of
sector size offset from the "disk", and in a bid to avoid Linux disk cache
inconsistency problem described by comment above, grub translates the address
again into the address offset from partition that has encompassed it, then use
that partition device in place of disk device.

The problem we encountered was that installing grub into multipath disk's
partition didn't work reliably. It boiled down to the disk cache problem
described above as strace result shown it was still using the whole disk
device, not the partition device we would expect.

This patch fixes the problem by adding the missing "/dev/dm-" name scheme
handling in grub_hostdisk_linux_find_partition(). After applying the patch
problem gets solved and we would like to have this fixing patch upstreamed as
it looks good material to be.

v1: Rework commit message.

Signed-off-by: Michael Chang <address@hidden>
 grub-core/osdep/linux/hostdisk.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/grub-core/osdep/linux/hostdisk.c b/grub-core/osdep/linux/hostdisk.c
index 06179fca7..ed530bdc4 100644
--- a/grub-core/osdep/linux/hostdisk.c
+++ b/grub-core/osdep/linux/hostdisk.c
@@ -263,6 +263,12 @@ grub_hostdisk_linux_find_partition (char *dev, 
grub_disk_addr_t sector)
       p = real_dev + len;
       format = "-part%d";
+  else if (strncmp (real_dev, "/dev/dm-",
+                   sizeof ("/dev/dm-") - 1) == 0)
+    {
+      p = real_dev + len - 1;
+      format = "%d";
+    }
   else if (real_dev[len - 1] >= '0' && real_dev[len - 1] <= '9')
       p = real_dev + len;

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