[PATCH v4 1/2] lvm: add lvm cache logical volume handling

[Michael Chang]

The lvm cache logical volume is the logical volume consisting of the original
and the cache pool logical volume. The original is usually on a larger and
slower storage device while the cache pool is on a smaller and faster one. The
performance of the original volume can be improved by storing the frequently
used data on the cache pool to utilize the greater performance of faster
device.

The default cache mode "writethrough" ensures that any data written will be
stored both in the cache and on the origin LV, therefore grub can be straight
to read the original lv as no data loss is guarenteed.

The second cache mode is "writeback", which delays writing from the cache pool
back to the origin LV to have increased performance. The drawback is potential
data loss if losing the associated cache device.

During the boot time grub reads the LVM offline i.e. LVM volumes are not
activated and mounted, hence it should be fine to read directly from original
lv since all cached data should have been flushed back in the process of taking
it offline.

It is also not much helpful to the situation by adding fsync calls to the
install code. The fsync did not force to write back dirty cache to the original
device and rather it would update associated cache metadata to complete the
write transaction with the cache device. IOW the writes to cached blocks still
go only to the cache device.

To write back dirty cache, as lvm cache did not support dirty cache flush per
block range, there'no way to do it for file. On the other hand the "cleaner"
policy is implemented and can be used to write back "all" dirty blocks in a
cache, which effectively drain all dirty cache gradually to attain and last in
the "clean" state, which can be useful for shrinking or decommissioning a
cache. The result and effect is not what we are looking for here.

In conclusion, as it seems no way to enforce file writes to the original
device, grub may suffer from power failure as it cannot assemble the cache
device and read the dirty data from it. However since the case is only
applicable to writeback mode which is sensitive to data lost in nature, I'd
still like to propose my (relatively simple) patch and treat reading dirty
cache as improvement.

Signed-off-by: Michael Chang <address@hidden>
---

Changes since v4:
 * Use grub_size_t for sz
 * Fix potential null pointer dereference in error path

Changes since v3:
 * Add some issue discussed in review process to commit message
 * Add note on LVM cache booting to grub manual
 * Some coding style change
 * Fix some problem in error handling

Changes since v2:
 * Move struct cache_lv definition to the beginning of file
 * Add handling when grub_(zalloc|malloc|strdup) etc failed

 grub-core/disk/lvm.c | 190 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 190 insertions(+)

diff --git a/grub-core/disk/lvm.c b/grub-core/disk/lvm.c
index 0cbd0dd16..e47be642a 100644
--- a/grub-core/disk/lvm.c
+++ b/grub-core/disk/lvm.c
@@ -33,6 +33,14 @@
 
 GRUB_MOD_LICENSE ("GPLv3+");
 
+struct cache_lv
+{
+  struct grub_diskfilter_lv *lv;
+  char *cache_pool;
+  char *origin;
+  struct cache_lv *next;
+};
+
 
 /* Go the string STR and return the number after STR.  *P will point
    at the number.  In case STR is not found, *P will be NULL and the
@@ -95,6 +103,34 @@ grub_lvm_check_flag (const char *p, const char *str, const 
char *flag)
     }
 }
 
+static void
+grub_lvm_free_cache_lvs (struct cache_lv *cache_lvs)
+{
+  struct cache_lv *cache;
+
+  while ((cache = cache_lvs))
+    {
+      cache_lvs = cache_lvs->next;
+
+      if (cache->lv)
+       {
+         unsigned int i;
+
+         for (i = 0; i < cache->lv->segment_count; ++i)
+           if (cache->lv->segments)
+             grub_free (cache->lv->segments[i].nodes);
+         grub_free (cache->lv->segments);
+         grub_free (cache->lv->fullname);
+         grub_free (cache->lv->idname);
+         grub_free (cache->lv->name);
+       }
+      grub_free (cache->lv);
+      grub_free (cache->origin);
+      grub_free (cache->cache_pool);
+      grub_free (cache);
+    }
+}
+
 static struct grub_diskfilter_vg * 
 grub_lvm_detect (grub_disk_t disk,
                 struct grub_diskfilter_pv_id *id,
@@ -243,6 +279,8 @@ grub_lvm_detect (grub_disk_t disk,
 
   if (! vg)
     {
+      struct cache_lv *cache_lvs = NULL;
+
       /* First time we see this volume group. We've to create the
         whole volume group structure. */
       vg = grub_malloc (sizeof (*vg));
@@ -672,6 +710,106 @@ grub_lvm_detect (grub_disk_t disk,
                          seg->nodes[seg->node_count - 1].name = tmp;
                        }
                    }
+                 else if (grub_memcmp (p, "cache\"",
+                                  sizeof ("cache\"") - 1) == 0)
+                   {
+                     struct cache_lv *cache = NULL;
+
+                     char *p2, *p3;
+                     grub_size_t sz;
+
+                     cache = grub_zalloc (sizeof (*cache));
+                     if (!cache)
+                       goto cache_lv_fail;
+                     cache->lv = grub_zalloc (sizeof (*cache->lv));
+                     if (!cache->lv)
+                       goto cache_lv_fail;
+                     grub_memcpy (cache->lv, lv, sizeof (*cache->lv));
+
+                     if (lv->fullname)
+                       {
+                         cache->lv->fullname = grub_strdup (lv->fullname);
+                         if (!cache->lv->fullname)
+                           goto cache_lv_fail;
+                       }
+                     if (lv->idname)
+                       {
+                         cache->lv->idname = grub_strdup (lv->idname);
+                         if (!cache->lv->idname)
+                           goto cache_lv_fail;
+                       }
+                     if (lv->name)
+                       {
+                         cache->lv->name = grub_strdup (lv->name);
+                         if (!cache->lv->name)
+                           goto cache_lv_fail;
+                       }
+
+                     skip_lv = 1;
+
+                     p2 = grub_strstr (p, "cache_pool = \"");
+                     if (!p2)
+                       goto cache_lv_fail;
+
+                     p2 = grub_strchr (p2, '"');
+                     if (!p2)
+                       goto cache_lv_fail;
+
+                     p3 = ++p2;
+                     p3 = grub_strchr (p3, '"');
+                     if (!p3)
+                       goto cache_lv_fail;
+
+                     sz = p3 - p2;
+
+                     cache->cache_pool = grub_malloc (sz + 1);
+                     if (!cache->cache_pool)
+                       goto cache_lv_fail;
+                     grub_memcpy (cache->cache_pool, p2, sz);
+                     cache->cache_pool[sz] = '\0';
+
+                     p2 = grub_strstr (p, "origin = \"");
+                     if (!p2)
+                       goto cache_lv_fail;
+
+                     p2 = grub_strchr (p2, '"');
+                     if (!p2)
+                       goto cache_lv_fail;
+
+                     p3 = ++p2;
+                     p3 = grub_strchr (p3, '"');
+                     if (!p3)
+                       goto cache_lv_fail;
+
+                     sz = p3 - p2;
+
+                     cache->origin = grub_malloc (sz + 1);
+                     if (!cache->origin)
+                       goto cache_lv_fail;
+                     grub_memcpy (cache->origin, p2, sz);
+                     cache->origin[sz] = '\0';
+
+                     cache->next = cache_lvs;
+                     cache_lvs = cache;
+                     break;
+
+                   cache_lv_fail:
+                     if (cache)
+                       {
+                         grub_free (cache->origin);
+                         grub_free (cache->cache_pool);
+                         if (cache->lv)
+                           {
+                             grub_free (cache->lv->fullname);
+                             grub_free (cache->lv->idname);
+                             grub_free (cache->lv->name);
+                           }
+                         grub_free (cache->lv);
+                         grub_free (cache);
+                       }
+                     grub_lvm_free_cache_lvs (cache_lvs);
+                     goto fail4;
+                   }
                  else
                    {
 #ifdef GRUB_UTIL
@@ -748,6 +886,58 @@ grub_lvm_detect (grub_disk_t disk,
              }
        
       }
+
+      {
+       struct cache_lv *cache;
+
+       for (cache = cache_lvs; cache; cache = cache->next)
+         {
+           struct grub_diskfilter_lv *lv;
+
+           for (lv = vg->lvs; lv; lv = lv->next)
+             if (grub_strcmp (lv->name, cache->origin) == 0)
+               break;
+           if (lv)
+             {
+               cache->lv->segments = grub_malloc (lv->segment_count * sizeof 
(*lv->segments));
+               if (!cache->lv->segments)
+                 {
+                   grub_lvm_free_cache_lvs (cache_lvs);
+                   goto fail4;
+                 }
+               grub_memcpy (cache->lv->segments, lv->segments, 
lv->segment_count * sizeof (*lv->segments));
+
+               for (i = 0; i < lv->segment_count; ++i)
+                 {
+                   struct grub_diskfilter_node *nodes = lv->segments[i].nodes;
+                   grub_size_t node_count = lv->segments[i].node_count;
+
+                   cache->lv->segments[i].nodes = grub_malloc (node_count * 
sizeof (*nodes));
+                   if (!cache->lv->segments[i].nodes)
+                     {
+                       for (j = 0; j < i; ++j)
+                         grub_free (cache->lv->segments[j].nodes);
+                       grub_free (cache->lv->segments);
+                       cache->lv->segments = NULL;
+                       grub_lvm_free_cache_lvs (cache_lvs);
+                       goto fail4;
+                     }
+                   grub_memcpy (cache->lv->segments[i].nodes, nodes, 
node_count * sizeof (*nodes));
+                 }
+
+               if (cache->lv->segments)
+                 {
+                   cache->lv->segment_count = lv->segment_count;
+                   cache->lv->vg = vg;
+                   cache->lv->next = vg->lvs;
+                   vg->lvs = cache->lv;
+                   cache->lv = NULL;
+                 }
+             }
+         }
+      }
+
+      grub_lvm_free_cache_lvs (cache_lvs);
       if (grub_diskfilter_vg_register (vg))
        goto fail4;
     }
-- 
2.16.4