[GFS2] Shrink gfs2_inode memory by half

Here is something I spotted (while looking for something entirely
different) the other day.

Rather than using a completion in each and every struct gfs2_holder,
this removes it in favour of hashed wait queues, thus saving a
considerable amount of memory both on the stack (where a number of
gfs2_holder structures are allocated) and in particular in the
gfs2_inode which has 8 gfs2_holder structures embedded within it.

As a result on x86_64 the gfs2_inode shrinks from 2488 bytes to
1912 bytes, a saving of 576 bytes per inode (no thats not a typo!).
In actual practice we get a much better result than that since
now that a gfs2_inode is under the 2048 byte barrier, we get two
per 4k slab page effectively halving the amount of memory required
to store gfs2_inodes.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

2 files changed, 44 insertions, 24 deletions

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index fb1960b7fdde..5341e03b873f 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -19,6 +19,7 @@
 #include <linux/gfs2_ondisk.h>
 #include <linux/list.h>
 #include <linux/lm_interface.h>
+#include <linux/wait.h>
 #include <asm/uaccess.h>
 
 #include "gfs2.h"
@@ -395,7 +396,6 @@ void gfs2_holder_init(struct gfs2_glock *gl, unsigned int state, unsigned flags,
         gh->gh_flags = flags;
         gh->gh_error = 0;
         gh->gh_iflags = 0;
-        init_completion(&gh->gh_wait);
 
         if (gh->gh_state == LM_ST_EXCLUSIVE)
                 gh->gh_flags |= GL_LOCAL_EXCL;
@@ -479,6 +479,29 @@ static void gfs2_holder_put(struct gfs2_holder *gh)
         kfree(gh);
 }
 
+static void gfs2_holder_dispose_or_wake(struct gfs2_holder *gh)
+{
+        if (test_bit(HIF_DEALLOC, &gh->gh_iflags)) {
+                gfs2_holder_put(gh);
+                return;
+        }
+        clear_bit(HIF_WAIT, &gh->gh_iflags);
+        smp_mb();
+        wake_up_bit(&gh->gh_iflags, HIF_WAIT);
+}
+
+static int holder_wait(void *word)
+{
+        schedule();
+        return 0;
+}
+
+static void wait_on_holder(struct gfs2_holder *gh)
+{
+        might_sleep();
+        wait_on_bit(&gh->gh_iflags, HIF_WAIT, holder_wait, TASK_UNINTERRUPTIBLE);
+}
+
 /**
  * rq_mutex - process a mutex request in the queue
  * @gh: the glock holder
@@ -493,7 +516,9 @@ static int rq_mutex(struct gfs2_holder *gh)
         list_del_init(&gh->gh_list);
         /*  gh->gh_error never examined.  */
         set_bit(GLF_LOCK, &gl->gl_flags);
-        complete(&gh->gh_wait);
+        clear_bit(HIF_WAIT, &gh->gh_flags);
+        smp_mb();
+        wake_up_bit(&gh->gh_iflags, HIF_WAIT);
 
         return 1;
 }
@@ -549,7 +574,7 @@ static int rq_promote(struct gfs2_holder *gh)
         gh->gh_error = 0;
         set_bit(HIF_HOLDER, &gh->gh_iflags);
 
-        complete(&gh->gh_wait);
+        gfs2_holder_dispose_or_wake(gh);
 
         return 0;
 }
@@ -573,10 +598,7 @@ static int rq_demote(struct gfs2_holder *gh)
                 list_del_init(&gh->gh_list);
                 gh->gh_error = 0;
                 spin_unlock(&gl->gl_spin);
-                if (test_bit(HIF_DEALLOC, &gh->gh_iflags))
-                        gfs2_holder_put(gh);
-                else
-                        complete(&gh->gh_wait);
+                gfs2_holder_dispose_or_wake(gh);
                 spin_lock(&gl->gl_spin);
         } else {
                 gl->gl_req_gh = gh;
@@ -684,6 +706,8 @@ static void gfs2_glmutex_lock(struct gfs2_glock *gl)
 
         gfs2_holder_init(gl, 0, 0, &gh);
         set_bit(HIF_MUTEX, &gh.gh_iflags);
+        if (test_and_set_bit(HIF_WAIT, &gh.gh_iflags))
+                BUG();
 
         spin_lock(&gl->gl_spin);
         if (test_and_set_bit(GLF_LOCK, &gl->gl_flags)) {
@@ -691,11 +715,13 @@ static void gfs2_glmutex_lock(struct gfs2_glock *gl)
         } else {
                 gl->gl_owner = current;
                 gl->gl_ip = (unsigned long)__builtin_return_address(0);
-                complete(&gh.gh_wait);
+                clear_bit(HIF_WAIT, &gh.gh_iflags);
+                smp_mb();
+                wake_up_bit(&gh.gh_iflags, HIF_WAIT);
         }
         spin_unlock(&gl->gl_spin);
 
-        wait_for_completion(&gh.gh_wait);
+        wait_on_holder(&gh);
         gfs2_holder_uninit(&gh);
 }
 
@@ -774,6 +800,7 @@ restart:
                         return;
                 set_bit(HIF_DEMOTE, &new_gh->gh_iflags);
                 set_bit(HIF_DEALLOC, &new_gh->gh_iflags);
+                set_bit(HIF_WAIT, &new_gh->gh_iflags);
 
                 goto restart;
         }
@@ -908,12 +935,8 @@ static void xmote_bh(struct gfs2_glock *gl, unsigned int ret)
 
         gfs2_glock_put(gl);
 
-        if (gh) {
-                if (test_bit(HIF_DEALLOC, &gh->gh_iflags))
-                        gfs2_holder_put(gh);
-                else
-                        complete(&gh->gh_wait);
-        }
+        if (gh)
+                gfs2_holder_dispose_or_wake(gh);
 }
 
 /**
@@ -999,12 +1022,8 @@ static void drop_bh(struct gfs2_glock *gl, unsigned int ret)
 
         gfs2_glock_put(gl);
 
-        if (gh) {
-                if (test_bit(HIF_DEALLOC, &gh->gh_iflags))
-                        gfs2_holder_put(gh);
-                else
-                        complete(&gh->gh_wait);
-        }
+        if (gh)
+                gfs2_holder_dispose_or_wake(gh);
 }
 
 /**
@@ -1105,8 +1124,7 @@ static int glock_wait_internal(struct gfs2_holder *gh)
         if (gh->gh_flags & LM_FLAG_PRIORITY)
                 do_cancels(gh);
 
-        wait_for_completion(&gh->gh_wait);
-
+        wait_on_holder(gh);
         if (gh->gh_error)
                 return gh->gh_error;
 
@@ -1162,6 +1180,8 @@ static void add_to_queue(struct gfs2_holder *gh)
         struct gfs2_holder *existing;
 
         BUG_ON(!gh->gh_owner);
+        if (test_and_set_bit(HIF_WAIT, &gh->gh_iflags))
+                BUG();
 
         existing = find_holder_by_owner(&gl->gl_holders, gh->gh_owner);
         if (existing) {
diff --git a/fs/gfs2/incore.h b/fs/gfs2/incore.h
index 9114851ac53e..a24c4af09ce0 100644
--- a/fs/gfs2/incore.h
+++ b/fs/gfs2/incore.h
@@ -128,6 +128,7 @@ enum {
         HIF_HOLDER              = 6,
         HIF_FIRST               = 7,
         HIF_ABORTED             = 9,
+        HIF_WAIT                = 10,
 };
 
 struct gfs2_holder {
@@ -140,7 +141,6 @@ struct gfs2_holder {
 
         int gh_error;
         unsigned long gh_iflags;
-        struct completion gh_wait;
         unsigned long gh_ip;
 };