ext4: Mark the unwritten buffer_head as mapped during write_begin

Setting BH_Unwritten buffer_heads as BH_Mapped avoids multiple
(unnecessary) calls to get_block() during the call to the write(2)
system call.  Setting BH_Unwritten buffer heads as BH_Mapped requires
that the writepages() functions can handle BH_Unwritten buffer_heads.

After this commit, things work as follows:

ext4_ext_get_block() returns unmapped, unwritten, buffer head when
called with create = 0 for prealloc space. This makes sure we handle
the read path and non-delayed allocation case correctly.  Even though
the buffer head is marked unmapped we have valid b_blocknr and b_bdev
values in the buffer_head.

ext4_da_get_block_prep() called for block resrevation will now return
mapped, unwritten, new buffer_head for prealloc space. This avoids
multiple calls to get_block() for write to same offset. By making such
buffers as BH_New, we also assure that sub-block zeroing of buffered
writes happens correctly.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index a953214..ea5c476 100644 (file)
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -2872,6 +2872,8 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
                        if (create == EXT4_CREATE_UNINITIALIZED_EXT)
                                goto out;
                        if (!create) {
+                               if (allocated > max_blocks)
+                                       allocated = max_blocks;
                                /*
                                 * We have blocks reserved already.  We
                                 * return allocated blocks so that delalloc
@@ -2879,8 +2881,6 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
                                 * the buffer head will be unmapped so that
                                 * a read from the block returns 0s.
                                 */
-                               if (allocated > max_blocks)
-                                       allocated = max_blocks;
                                set_buffer_unwritten(bh_result);
                                bh_result->b_bdev = inode->i_sb->s_bdev;
                                bh_result->b_blocknr = newblock;

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index d7ad0bb..96f3366 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1852,7 +1852,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd)
  * @logical - first logical block to start assignment with
  *
  * the function goes through all passed space and put actual disk
- * block numbers into buffer heads, dropping BH_Delay
+ * block numbers into buffer heads, dropping BH_Delay and BH_Unwritten
  */
 static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, sector_t logical,
                                 struct buffer_head *exbh)
@@ -1902,16 +1902,24 @@ static void mpage_put_bnr_to_bhs(struct mpage_da_data *mpd, sector_t logical,
                        do {
                                if (cur_logical >= logical + blocks)
                                        break;
-                               if (buffer_delay(bh)) {
-                                       bh->b_blocknr = pblock;
-                                       clear_buffer_delay(bh);
-                                       bh->b_bdev = inode->i_sb->s_bdev;
-                               } else if (buffer_unwritten(bh)) {
-                                       bh->b_blocknr = pblock;
-                                       clear_buffer_unwritten(bh);
-                                       set_buffer_mapped(bh);
-                                       set_buffer_new(bh);
-                                       bh->b_bdev = inode->i_sb->s_bdev;
+
+                               if (buffer_delay(bh) ||
+                                               buffer_unwritten(bh)) {
+
+                                       BUG_ON(bh->b_bdev != inode->i_sb->s_bdev);
+
+                                       if (buffer_delay(bh)) {
+                                               clear_buffer_delay(bh);
+                                               bh->b_blocknr = pblock;
+                                       } else {
+                                               /*
+                                                * unwritten already should have
+                                                * blocknr assigned. Verify that
+                                                */
+                                               clear_buffer_unwritten(bh);
+                                               BUG_ON(bh->b_blocknr != pblock);
+                                       }
+
                                } else if (buffer_mapped(bh))
                                        BUG_ON(bh->b_blocknr != pblock);
 
@@ -2053,7 +2061,8 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
         * We consider only non-mapped and non-allocated blocks
         */
        if ((mpd->b_state  & (1 << BH_Mapped)) &&
-           !(mpd->b_state & (1 << BH_Delay)))
+               !(mpd->b_state & (1 << BH_Delay)) &&
+               !(mpd->b_state & (1 << BH_Unwritten)))
                return 0;
        /*
         * We need to make sure the BH_Delay flag is passed down to
@@ -2205,6 +2214,17 @@ flush_it:
        return;
 }
 
+static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
+{
+       /*
+        * unmapped buffer is possible for holes.
+        * delay buffer is possible with delayed allocation.
+        * We also need to consider unwritten buffer as unmapped.
+        */
+       return (!buffer_mapped(bh) || buffer_delay(bh) ||
+                               buffer_unwritten(bh)) && buffer_dirty(bh);
+}
+
 /*
  * __mpage_da_writepage - finds extent of pages and blocks
  *
@@ -2289,8 +2309,7 @@ static int __mpage_da_writepage(struct page *page,
                         * Otherwise we won't make progress
                         * with the page in ext4_da_writepage
                         */
-                       if (buffer_dirty(bh) &&
-                           (!buffer_mapped(bh) || buffer_delay(bh))) {
+                       if (ext4_bh_unmapped_or_delay(NULL, bh)) {
                                mpage_add_bh_to_extent(mpd, logical,
                                                       bh->b_size,
                                                       bh->b_state);
@@ -2318,6 +2337,14 @@ static int __mpage_da_writepage(struct page *page,
 /*
  * this is a special callback for ->write_begin() only
  * it's intention is to return mapped block or reserve space
+ *
+ * For delayed buffer_head we have BH_Mapped, BH_New, BH_Delay set.
+ * We also have b_blocknr = -1 and b_bdev initialized properly
+ *
+ * For unwritten buffer_head we have BH_Mapped, BH_New, BH_Unwritten set.
+ * We also have b_blocknr = physicalblock mapping unwritten extent and b_bdev
+ * initialized properly.
+ *
  */
 static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
                                  struct buffer_head *bh_result, int create)
@@ -2353,28 +2380,23 @@ static int ext4_da_get_block_prep(struct inode *inode, sector_t iblock,
                set_buffer_delay(bh_result);
        } else if (ret > 0) {
                bh_result->b_size = (ret << inode->i_blkbits);
-               /*
-                * With sub-block writes into unwritten extents
-                * we also need to mark the buffer as new so that
-                * the unwritten parts of the buffer gets correctly zeroed.
-                */
-               if (buffer_unwritten(bh_result))
+               if (buffer_unwritten(bh_result)) {
+                       /* A delayed write to unwritten bh should
+                        * be marked new and mapped.  Mapped ensures
+                        * that we don't do get_block multiple times
+                        * when we write to the same offset and new
+                        * ensures that we do proper zero out for
+                        * partial write.
+                        */
                        set_buffer_new(bh_result);
+                       set_buffer_mapped(bh_result);
+               }
                ret = 0;
        }
 
        return ret;
 }
 
-static int ext4_bh_unmapped_or_delay(handle_t *handle, struct buffer_head *bh)
-{
-       /*
-        * unmapped buffer is possible for holes.
-        * delay buffer is possible with delayed allocation
-        */
-       return ((!buffer_mapped(bh) || buffer_delay(bh)) && buffer_dirty(bh));
-}
-
 static int ext4_normal_get_block_write(struct inode *inode, sector_t iblock,
                                   struct buffer_head *bh_result, int create)
 {
@@ -2828,7 +2850,7 @@ static int ext4_da_should_update_i_disksize(struct page *page,
        for (i = 0; i < idx; i++)
                bh = bh->b_this_page;
 
-       if (!buffer_mapped(bh) || (buffer_delay(bh)))
+       if (!buffer_mapped(bh) || (buffer_delay(bh)) || buffer_unwritten(bh))
                return 0;
        return 1;
 }