[PATCH] no-buffer-head ext2 option

Implements a new set of block address_space_operations which will never
attach buffer_heads to file pagecache.  These can be turned on for ext2
with the `nobh' mount option.

During write-intensive testing on a 7G machine, total buffer_head
storage remained below 0.3 megabytes.  And those buffer_heads are
against ZONE_NORMAL pagecache and will be reclaimed by ZONE_NORMAL
memory pressure.

This work is, of course, a special for the huge highmem machines.
Possibly it obsoletes the buffer_heads_over_limit stuff (which doesn't
work terribly well), but that code is simple, and will provide relief
for other filesystems.


It should be noted that the nobh_prepare_write() function and the
PageMappedToDisk() infrastructure is what is needed to solve the
problem of user data corruption when the filesystem which backs a
sparse MAP_SHARED mapping runs out of space.  We can use this code in
filemap_nopage() to ensure that all mapped pages have space allocated
on-disk.  Deliver SIGBUS on ENOSPC.

This will require a new address_space op, I expect.

3 files changed, 9 insertions, 0 deletions

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 8587dd6f7146..4e7a9bbf99dd 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -190,6 +190,9 @@ sector_t generic_block_bmap(struct address_space *, sector_t, get_block_t *);
 int generic_commit_write(struct file *, struct page *, unsigned, unsigned);
 int block_truncate_page(struct address_space *, loff_t, get_block_t *);
 int file_fsync(struct file *, struct dentry *, int);
+int nobh_prepare_write(struct page*, unsigned, unsigned, get_block_t*);
+int nobh_commit_write(struct file *, struct page *, unsigned, unsigned);
+int nobh_truncate_page(struct address_space *, loff_t);
 
 #define OSYNC_METADATA	(1<<0)
 #define OSYNC_DATA	(1<<1)
diff --git a/include/linux/ext2_fs.h b/include/linux/ext2_fs.h
index 0d008cfb99a8..d701ba88c688 100644
--- a/include/linux/ext2_fs.h
+++ b/include/linux/ext2_fs.h
@@ -308,6 +308,7 @@ struct ext2_inode {
 #define EXT2_MOUNT_ERRORS_RO		0x0020	/* Remount fs ro on errors */
 #define EXT2_MOUNT_ERRORS_PANIC		0x0040	/* Panic on errors */
 #define EXT2_MOUNT_MINIX_DF		0x0080	/* Mimics the Minix statfs */
+#define EXT2_MOUNT_NOBH			0x0100	/* No buffer_heads */
 #define EXT2_MOUNT_NO_UID32		0x0200  /* Disable 32-bit UIDs */
 #define EXT2_MOUNT_XATTR_USER		0x4000	/* Extended user attributes */
 #define EXT2_MOUNT_POSIX_ACL		0x8000	/* POSIX Access Control Lists */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 44480d80952f..f02449871c07 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -70,6 +70,7 @@
 #define PG_chainlock		15	/* lock bit for ->pte_chain */
 
 #define PG_direct		16	/* ->pte_chain points directly at pte */
+#define PG_mappedtodisk		17	/* Has blocks allocated on-disk */
 
 /*
  * Global page accounting.  One instance per CPU.  Only unsigned longs are
@@ -233,6 +234,10 @@ extern void get_full_page_state(struct page_state *ret);
 #define ClearPageDirect(page)		clear_bit(PG_direct, &(page)->flags)
 #define TestClearPageDirect(page)	test_and_clear_bit(PG_direct, &(page)->flags)
 
+#define PageMappedToDisk(page)	test_bit(PG_mappedtodisk, &(page)->flags)
+#define SetPageMappedToDisk(page) set_bit(PG_mappedtodisk, &(page)->flags)
+#define ClearPageMappedToDisk(page) clear_bit(PG_mappedtodisk, &(page)->flags)
+
 /*
  * The PageSwapCache predicate doesn't use a PG_flag at this time,
  * but it may again do so one day.