[PATCH] Shrinks sizeof(files_struct) and better layout

1) Reduce the size of (struct fdtable) to exactly 64 bytes on 32bits
   platforms, lowering kmalloc() allocated space by 50%.

2) Reduce the size of (files_struct), using a special 32 bits (or
   64bits) embedded_fd_set, instead of a 1024 bits fd_set for the
   close_on_exec_init and open_fds_init fields.  This save some ram (248
   bytes per task) as most tasks dont open more than 32 files.  D-Cache
   footprint for such tasks is also reduced to the minimum.

3) Reduce size of allocated fdset.  Currently two full pages are
   allocated, that is 32768 bits on x86 for example, and way too much.  The
   minimum is now L1_CACHE_BYTES.

UP and SMP should benefit from this patch, because most tasks will touch
only one cache line when open()/close() stdin/stdout/stderr (0/1/2),
(next_fd, close_on_exec_init, open_fds_init, fd_array[0 ..  2] being in the
same cache line)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 4 insertions, 5 deletions

diff --git a/fs/fcntl.c b/fs/fcntl.c
index dc4a7007f4e7..03c789560fb8 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -73,8 +73,8 @@ repeat:
          * orig_start..fdt->next_fd
          */
         start = orig_start;
-        if (start < fdt->next_fd)
-                start = fdt->next_fd;
+        if (start < files->next_fd)
+                start = files->next_fd;
 
         newfd = start;
         if (start < fdt->max_fdset) {
@@ -102,9 +102,8 @@ repeat:
          * we reacquire the fdtable pointer and use it while holding
          * the lock, no one can free it during that time.
          */
-        fdt = files_fdtable(files);
-        if (start <= fdt->next_fd)
-                fdt->next_fd = newfd + 1;
+        if (start <= files->next_fd)
+                files->next_fd = newfd + 1;
 
         error = newfd;