fs/buffer.c: make block-size be per-page and protected by the page lock

This makes the buffer size handling be a per-page thing, which allows us
to not have to worry about locking too much when changing the buffer
size.  If a page doesn't have buffers, we still need to read the block
size from the inode, but we can do that with ACCESS_ONCE(), so that even
if the size is changing, we get a consistent value.

This doesn't convert all functions - many of the buffer functions are
used purely by filesystems, which in turn results in the buffer size
being fixed at mount-time.  So they don't have the same consistency
issues that the raw device access can have.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 48 insertions, 31 deletions

diff --git a/fs/buffer.c b/fs/buffer.c
index b5f044283edb..28a74ff5324b 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1553,6 +1553,28 @@ void unmap_underlying_metadata(struct block_device *bdev, sector_t block)
 EXPORT_SYMBOL(unmap_underlying_metadata);
 
 /*
+ * Size is a power-of-two in the range 512..PAGE_SIZE,
+ * and the case we care about most is PAGE_SIZE.
+ *
+ * So this *could* possibly be written with those
+ * constraints in mind (relevant mostly if some
+ * architecture has a slow bit-scan instruction)
+ */
+static inline int block_size_bits(unsigned int blocksize)
+{
+        return ilog2(blocksize);
+}
+
+static struct buffer_head *create_page_buffers(struct page *page, struct inode *inode, unsigned int b_state)
+{
+        BUG_ON(!PageLocked(page));
+
+        if (!page_has_buffers(page))
+                create_empty_buffers(page, 1 << ACCESS_ONCE(inode->i_blkbits), b_state);
+        return page_buffers(page);
+}
+
+/*
  * NOTE! All mapped/uptodate combinations are valid:
  *
  *      Mapped  Uptodate        Meaning
@@ -1589,19 +1611,13 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
         sector_t block;
         sector_t last_block;
         struct buffer_head *bh, *head;
-        const unsigned blocksize = 1 << inode->i_blkbits;
+        unsigned int blocksize, bbits;
         int nr_underway = 0;
         int write_op = (wbc->sync_mode == WB_SYNC_ALL ?
                         WRITE_SYNC : WRITE);
 
-        BUG_ON(!PageLocked(page));
-
-        last_block = (i_size_read(inode) - 1) >> inode->i_blkbits;
-
-        if (!page_has_buffers(page)) {
-                create_empty_buffers(page, blocksize,
+        head = create_page_buffers(page, inode,
                                         (1 << BH_Dirty)|(1 << BH_Uptodate));
-        }
 
         /*
          * Be very careful.  We have no exclusion from __set_page_dirty_buffers
@@ -1613,9 +1629,12 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
          * handle that here by just cleaning them.
          */
 
-        block = (sector_t)page->index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
-        head = page_buffers(page);
         bh = head;
+        blocksize = bh->b_size;
+        bbits = block_size_bits(blocksize);
+
+        block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
+        last_block = (i_size_read(inode) - 1) >> bbits;
 
         /*
          * Get all the dirty buffers mapped to disk addresses and
@@ -1806,12 +1825,10 @@ int __block_write_begin(struct page *page, loff_t pos, unsigned len,
         BUG_ON(to > PAGE_CACHE_SIZE);
         BUG_ON(from > to);
 
-        blocksize = 1 << inode->i_blkbits;
-        if (!page_has_buffers(page))
-                create_empty_buffers(page, blocksize, 0);
-        head = page_buffers(page);
+        head = create_page_buffers(page, inode, 0);
+        blocksize = head->b_size;
+        bbits = block_size_bits(blocksize);
 
-        bbits = inode->i_blkbits;
         block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
 
         for(bh = head, block_start = 0; bh != head || !block_start;
@@ -1881,11 +1898,11 @@ static int __block_commit_write(struct inode *inode, struct page *page,
         unsigned blocksize;
         struct buffer_head *bh, *head;
 
-        blocksize = 1 << inode->i_blkbits;
+        bh = head = page_buffers(page);
+        blocksize = bh->b_size;
 
-        for(bh = head = page_buffers(page), block_start = 0;
-            bh != head || !block_start;
-            block_start=block_end, bh = bh->b_this_page) {
+        block_start = 0;
+        do {
                 block_end = block_start + blocksize;
                 if (block_end <= from || block_start >= to) {
                         if (!buffer_uptodate(bh))
@@ -1895,7 +1912,10 @@ static int __block_commit_write(struct inode *inode, struct page *page,
                         mark_buffer_dirty(bh);
                 }
                 clear_buffer_new(bh);
-        }
+
+                block_start = block_end;
+                bh = bh->b_this_page;
+        } while (bh != head);
 
         /*
          * If this is a partial write which happened to make all buffers
@@ -2020,7 +2040,6 @@ EXPORT_SYMBOL(generic_write_end);
 int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
                                         unsigned long from)
 {
-        struct inode *inode = page->mapping->host;
         unsigned block_start, block_end, blocksize;
         unsigned to;
         struct buffer_head *bh, *head;
@@ -2029,13 +2048,13 @@ int block_is_partially_uptodate(struct page *page, read_descriptor_t *desc,
         if (!page_has_buffers(page))
                 return 0;
 
-        blocksize = 1 << inode->i_blkbits;
+        head = page_buffers(page);
+        blocksize = head->b_size;
         to = min_t(unsigned, PAGE_CACHE_SIZE - from, desc->count);
         to = from + to;
         if (from < blocksize && to > PAGE_CACHE_SIZE - blocksize)
                 return 0;
 
-        head = page_buffers(page);
         bh = head;
         block_start = 0;
         do {
@@ -2068,18 +2087,16 @@ int block_read_full_page(struct page *page, get_block_t *get_block)
         struct inode *inode = page->mapping->host;
         sector_t iblock, lblock;
         struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
-        unsigned int blocksize;
+        unsigned int blocksize, bbits;
         int nr, i;
         int fully_mapped = 1;
 
-        BUG_ON(!PageLocked(page));
-        blocksize = 1 << inode->i_blkbits;
-        if (!page_has_buffers(page))
-                create_empty_buffers(page, blocksize, 0);
-        head = page_buffers(page);
+        head = create_page_buffers(page, inode, 0);
+        blocksize = head->b_size;
+        bbits = block_size_bits(blocksize);
 
-        iblock = (sector_t)page->index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
-        lblock = (i_size_read(inode)+blocksize-1) >> inode->i_blkbits;
+        iblock = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
+        lblock = (i_size_read(inode)+blocksize-1) >> bbits;
         bh = head;
         nr = 0;
         i = 0;