ext4: Save stack space by removing fake buffer heads

Struct mpage_da_data and mpage_add_bh_to_extent() use a fake struct
buffer_head which is 104 bytes on an x86_64 system, but only use 24
bytes of the structure.  On systems that use a spinlock for atomic_t,
the stack savings will be even greater.

It turns out that using a fake struct buffer_head doesn't even save
that much code, and it makes the code more confusing since it's not
used as a "real" buffer head.  So just store pass b_size and b_state
in mpage_add_bh_to_extent(), and store b_size, b_state, and b_block_nr
in the mpage_da_data structure.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 39 insertions, 44 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 924ba8afc227..008b28a859d0 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1703,7 +1703,9 @@ static void ext4_da_page_release_reservation(struct page *page,
 
 struct mpage_da_data {
         struct inode *inode;
-        struct buffer_head lbh;                 /* extent of blocks */
+        sector_t b_blocknr;             /* start block number of extent */
+        size_t b_size;                  /* size of extent */
+        unsigned long b_state;          /* state of the extent */
         unsigned long first_page, next_page;    /* extent of pages */
         struct writeback_control *wbc;
         int io_done;
@@ -1737,7 +1739,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd)
         /*
          * We need to start from the first_page to the next_page - 1
          * to make sure we also write the mapped dirty buffer_heads.
-         * If we look at mpd->lbh.b_blocknr we would only be looking
+         * If we look at mpd->b_blocknr we would only be looking
          * at the currently mapped buffer_heads.
          */
         index = mpd->first_page;
@@ -1975,7 +1977,7 @@ static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
 /*
  * mpage_da_map_blocks - go through given space
  *
- * @mpd->lbh - bh describing space
+ * @mpd - bh describing space
  *
  * The function skips space we know is already mapped to disk blocks.
  *
@@ -1984,18 +1986,18 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
 {
         int err = 0;
         struct buffer_head new;
-        struct buffer_head *lbh = &mpd->lbh;
         sector_t next;
 
         /*
          * We consider only non-mapped and non-allocated blocks
          */
-        if (buffer_mapped(lbh) && !buffer_delay(lbh))
+        if ((mpd->b_state  & (1 << BH_Mapped)) &&
+            !(mpd->b_state & (1 << BH_Delay)))
                 return 0;
-        new.b_state = lbh->b_state;
+        new.b_state = mpd->b_state;
         new.b_blocknr = 0;
-        new.b_size = lbh->b_size;
-        next = lbh->b_blocknr;
+        new.b_size = mpd->b_size;
+        next = mpd->b_blocknr;
         /*
          * If we didn't accumulate anything
          * to write simply return
@@ -2031,7 +2033,7 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
                                   "%zd with error %d\n",
                                   __func__, mpd->inode->i_ino,
                                   (unsigned long long)next,
-                                  lbh->b_size >> mpd->inode->i_blkbits, err);
+                                  mpd->b_size >> mpd->inode->i_blkbits, err);
                 printk(KERN_EMERG "This should not happen.!! "
                                         "Data will be lost\n");
                 if (err == -ENOSPC) {
@@ -2039,7 +2041,7 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
                 }
                 /* invlaidate all the pages */
                 ext4_da_block_invalidatepages(mpd, next,
-                                lbh->b_size >> mpd->inode->i_blkbits);
+                                mpd->b_size >> mpd->inode->i_blkbits);
                 return err;
         }
         BUG_ON(new.b_size == 0);
@@ -2051,7 +2053,8 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
          * If blocks are delayed marked, we need to
          * put actual blocknr and drop delayed bit
          */
-        if (buffer_delay(lbh) || buffer_unwritten(lbh))
+        if ((mpd->b_state & (1 << BH_Delay)) ||
+            (mpd->b_state & (1 << BH_Unwritten)))
                 mpage_put_bnr_to_bhs(mpd, next, &new);
 
         return 0;
@@ -2070,12 +2073,11 @@ static int mpage_da_map_blocks(struct mpage_da_data *mpd)
  * the function is used to collect contig. blocks in same state
  */
 static void mpage_add_bh_to_extent(struct mpage_da_data *mpd,
-                                   sector_t logical, struct buffer_head *bh)
+                                   sector_t logical, size_t b_size,
+                                   unsigned long b_state)
 {
         sector_t next;
-        size_t b_size = bh->b_size;
-        struct buffer_head *lbh = &mpd->lbh;
-        int nrblocks = lbh->b_size >> mpd->inode->i_blkbits;
+        int nrblocks = mpd->b_size >> mpd->inode->i_blkbits;
 
         /* check if thereserved journal credits might overflow */
         if (!(EXT4_I(mpd->inode)->i_flags & EXT4_EXTENTS_FL)) {
@@ -2102,19 +2104,19 @@ static void mpage_add_bh_to_extent(struct mpage_da_data *mpd,
         /*
          * First block in the extent
          */
-        if (lbh->b_size == 0) {
-                lbh->b_blocknr = logical;
-                lbh->b_size = b_size;
-                lbh->b_state = bh->b_state & BH_FLAGS;
+        if (mpd->b_size == 0) {
+                mpd->b_blocknr = logical;
+                mpd->b_size = b_size;
+                mpd->b_state = b_state & BH_FLAGS;
                 return;
         }
 
-        next = lbh->b_blocknr + nrblocks;
+        next = mpd->b_blocknr + nrblocks;
         /*
          * Can we merge the block to our big extent?
          */
-        if (logical == next && (bh->b_state & BH_FLAGS) == lbh->b_state) {
-                lbh->b_size += b_size;
+        if (logical == next && (b_state & BH_FLAGS) == mpd->b_state) {
+                mpd->b_size += b_size;
                 return;
         }
 
@@ -2143,7 +2145,7 @@ static int __mpage_da_writepage(struct page *page,
 {
         struct mpage_da_data *mpd = data;
         struct inode *inode = mpd->inode;
-        struct buffer_head *bh, *head, fake;
+        struct buffer_head *bh, *head;
         sector_t logical;
 
         if (mpd->io_done) {
@@ -2185,9 +2187,9 @@ static int __mpage_da_writepage(struct page *page,
                 /*
                  * ... and blocks
                  */
-                mpd->lbh.b_size = 0;
-                mpd->lbh.b_state = 0;
-                mpd->lbh.b_blocknr = 0;
+                mpd->b_size = 0;
+                mpd->b_state = 0;
+                mpd->b_blocknr = 0;
         }
 
         mpd->next_page = page->index + 1;
@@ -2195,16 +2197,8 @@ static int __mpage_da_writepage(struct page *page,
                   (PAGE_CACHE_SHIFT - inode->i_blkbits);
 
         if (!page_has_buffers(page)) {
-                /*
-                 * There is no attached buffer heads yet (mmap?)
-                 * we treat the page asfull of dirty blocks
-                 */
-                bh = &fake;
-                bh->b_size = PAGE_CACHE_SIZE;
-                bh->b_state = 0;
-                set_buffer_dirty(bh);
-                set_buffer_uptodate(bh);
-                mpage_add_bh_to_extent(mpd, logical, bh);
+                mpage_add_bh_to_extent(mpd, logical, PAGE_CACHE_SIZE,
+                                       (1 << BH_Dirty) | (1 << BH_Uptodate));
                 if (mpd->io_done)
                         return MPAGE_DA_EXTENT_TAIL;
         } else {
@@ -2222,8 +2216,10 @@ static int __mpage_da_writepage(struct page *page,
                          * with the page in ext4_da_writepage
                          */
                         if (buffer_dirty(bh) &&
-                                (!buffer_mapped(bh) || buffer_delay(bh))) {
-                                mpage_add_bh_to_extent(mpd, logical, bh);
+                            (!buffer_mapped(bh) || buffer_delay(bh))) {
+                                mpage_add_bh_to_extent(mpd, logical,
+                                                       bh->b_size,
+                                                       bh->b_state);
                                 if (mpd->io_done)
                                         return MPAGE_DA_EXTENT_TAIL;
                         } else if (buffer_dirty(bh) && (buffer_mapped(bh))) {
@@ -2235,9 +2231,8 @@ static int __mpage_da_writepage(struct page *page,
                                  * unmapped buffer_head later we need to
                                  * use the b_state flag of that buffer_head.
                                  */
-                                if (mpd->lbh.b_size == 0)
-                                        mpd->lbh.b_state =
-                                                bh->b_state & BH_FLAGS;
+                                if (mpd->b_size == 0)
+                                        mpd->b_state = bh->b_state & BH_FLAGS;
                         }
                         logical++;
                 } while ((bh = bh->b_this_page) != head);
@@ -2263,9 +2258,9 @@ static int mpage_da_writepages(struct address_space *mapping,
 {
         int ret;
 
-        mpd->lbh.b_size = 0;
-        mpd->lbh.b_state = 0;
-        mpd->lbh.b_blocknr = 0;
+        mpd->b_size = 0;
+        mpd->b_state = 0;
+        mpd->b_blocknr = 0;
         mpd->first_page = 0;
         mpd->next_page = 0;
         mpd->io_done = 0;