2 files changed, 235 insertions, 0 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 5c38120c389c..ccfa81f33bb0 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -529,6 +529,11 @@ struct ext4_new_group_data {
 #define EXT4_FREE_BLOCKS_NO_QUOT_UPDATE 0x0008
 
 /*
+ * Flags used by ext4_discard_partial_page_buffers
+ */
+#define EXT4_DISCARD_PARTIAL_PG_ZERO_UNMAPPED   0x0001
+
+/*
  * ioctl commands
  */
 #define EXT4_IOC_GETFLAGS               FS_IOC_GETFLAGS
@@ -1838,6 +1843,12 @@ extern int ext4_block_truncate_page(handle_t *handle,
                 struct address_space *mapping, loff_t from);
 extern int ext4_block_zero_page_range(handle_t *handle,
                 struct address_space *mapping, loff_t from, loff_t length);
+extern int ext4_discard_partial_page_buffers(handle_t *handle,
+                struct address_space *mapping, loff_t from,
+                loff_t length, int flags);
+extern int ext4_discard_partial_page_buffers_no_lock(handle_t *handle,
+                struct inode *inode, struct page *page, loff_t from,
+                loff_t length, int flags);
 extern int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf);
 extern qsize_t *ext4_get_reserved_space(struct inode *inode);
 extern void ext4_da_update_reserve_space(struct inode *inode,
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b84f127c085d..d1b1ef71e5b7 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2966,6 +2966,230 @@ void ext4_set_aops(struct inode *inode)
                 inode->i_mapping->a_ops = &ext4_journalled_aops;
 }
 
+
+/*
+ * ext4_discard_partial_page_buffers()
+ * Wrapper function for ext4_discard_partial_page_buffers_no_lock.
+ * This function finds and locks the page containing the offset
+ * "from" and passes it to ext4_discard_partial_page_buffers_no_lock.
+ * Calling functions that already have the page locked should call
+ * ext4_discard_partial_page_buffers_no_lock directly.
+ */
+int ext4_discard_partial_page_buffers(handle_t *handle,
+                struct address_space *mapping, loff_t from,
+                loff_t length, int flags)
+{
+        struct inode *inode = mapping->host;
+        struct page *page;
+        int err = 0;
+
+        page = find_or_create_page(mapping, from >> PAGE_CACHE_SHIFT,
+                                   mapping_gfp_mask(mapping) & ~__GFP_FS);
+        if (!page)
+                return -EINVAL;
+
+        err = ext4_discard_partial_page_buffers_no_lock(handle, inode, page,
+                from, length, flags);
+
+        unlock_page(page);
+        page_cache_release(page);
+        return err;
+}
+
+/*
+ * ext4_discard_partial_page_buffers_no_lock()
+ * Zeros a page range of length 'length' starting from offset 'from'.
+ * Buffer heads that correspond to the block aligned regions of the
+ * zeroed range will be unmapped.  Unblock aligned regions
+ * will have the corresponding buffer head mapped if needed so that
+ * that region of the page can be updated with the partial zero out.
+ *
+ * This function assumes that the page has already been  locked.  The
+ * The range to be discarded must be contained with in the given page.
+ * If the specified range exceeds the end of the page it will be shortened
+ * to the end of the page that corresponds to 'from'.  This function is
+ * appropriate for updating a page and it buffer heads to be unmapped and
+ * zeroed for blocks that have been either released, or are going to be
+ * released.
+ *
+ * handle: The journal handle
+ * inode:  The files inode
+ * page:   A locked page that contains the offset "from"
+ * from:   The starting byte offset (from the begining of the file)
+ *         to begin discarding
+ * len:    The length of bytes to discard
+ * flags:  Optional flags that may be used:
+ *
+ *         EXT4_DISCARD_PARTIAL_PG_ZERO_UNMAPPED
+ *         Only zero the regions of the page whose buffer heads
+ *         have already been unmapped.  This flag is appropriate
+ *         for updateing the contents of a page whose blocks may
+ *         have already been released, and we only want to zero
+ *         out the regions that correspond to those released blocks.
+ *
+ * Returns zero on sucess or negative on failure.
+ */
+int ext4_discard_partial_page_buffers_no_lock(handle_t *handle,
+                struct inode *inode, struct page *page, loff_t from,
+                loff_t length, int flags)
+{
+        ext4_fsblk_t index = from >> PAGE_CACHE_SHIFT;
+        unsigned int offset = from & (PAGE_CACHE_SIZE-1);
+        unsigned int blocksize, max, pos;
+        unsigned int end_of_block, range_to_discard;
+        ext4_lblk_t iblock;
+        struct buffer_head *bh;
+        int err = 0;
+
+        blocksize = inode->i_sb->s_blocksize;
+        max = PAGE_CACHE_SIZE - offset;
+
+        if (index != page->index)
+                return -EINVAL;
+
+        /*
+         * correct length if it does not fall between
+         * 'from' and the end of the page
+         */
+        if (length > max || length < 0)
+                length = max;
+
+        iblock = index << (PAGE_CACHE_SHIFT - inode->i_sb->s_blocksize_bits);
+
+        if (!page_has_buffers(page)) {
+                /*
+                 * If the range to be discarded covers a partial block
+                 * we need to get the page buffers.  This is because
+                 * partial blocks cannot be released and the page needs
+                 * to be updated with the contents of the block before
+                 * we write the zeros on top of it.
+                 */
+                if (!(from & (blocksize - 1)) ||
+                    !((from + length) & (blocksize - 1))) {
+                        create_empty_buffers(page, blocksize, 0);
+                } else {
+                        /*
+                         * If there are no partial blocks,
+                         * there is nothing to update,
+                         * so we can return now
+                         */
+                        return 0;
+                }
+        }
+
+        /* Find the buffer that contains "offset" */
+        bh = page_buffers(page);
+        pos = blocksize;
+        while (offset >= pos) {
+                bh = bh->b_this_page;
+                iblock++;
+                pos += blocksize;
+        }
+
+        pos = offset;
+        while (pos < offset + length) {
+                err = 0;
+
+                /* The length of space left to zero and unmap */
+                range_to_discard = offset + length - pos;
+
+                /* The length of space until the end of the block */
+                end_of_block = blocksize - (pos & (blocksize-1));
+
+                /*
+                 * Do not unmap or zero past end of block
+                 * for this buffer head
+                 */
+                if (range_to_discard > end_of_block)
+                        range_to_discard = end_of_block;
+
+
+                /*
+                 * Skip this buffer head if we are only zeroing unampped
+                 * regions of the page
+                 */
+                if (flags & EXT4_DISCARD_PARTIAL_PG_ZERO_UNMAPPED &&
+                        buffer_mapped(bh))
+                                goto next;
+
+                /* If the range is block aligned, unmap */
+                if (range_to_discard == blocksize) {
+                        clear_buffer_dirty(bh);
+                        bh->b_bdev = NULL;
+                        clear_buffer_mapped(bh);
+                        clear_buffer_req(bh);
+                        clear_buffer_new(bh);
+                        clear_buffer_delay(bh);
+                        clear_buffer_unwritten(bh);
+                        clear_buffer_uptodate(bh);
+                        zero_user(page, pos, range_to_discard);
+                        BUFFER_TRACE(bh, "Buffer discarded");
+                        goto next;
+                }
+
+                /*
+                 * If this block is not completely contained in the range
+                 * to be discarded, then it is not going to be released. Because
+                 * we need to keep this block, we need to make sure this part
+                 * of the page is uptodate before we modify it by writeing
+                 * partial zeros on it.
+                 */
+                if (!buffer_mapped(bh)) {
+                        /*
+                         * Buffer head must be mapped before we can read
+                         * from the block
+                         */
+                        BUFFER_TRACE(bh, "unmapped");
+                        ext4_get_block(inode, iblock, bh, 0);
+                        /* unmapped? It's a hole - nothing to do */
+                        if (!buffer_mapped(bh)) {
+                                BUFFER_TRACE(bh, "still unmapped");
+                                goto next;
+                        }
+                }
+
+                /* Ok, it's mapped. Make sure it's up-to-date */
+                if (PageUptodate(page))
+                        set_buffer_uptodate(bh);
+
+                if (!buffer_uptodate(bh)) {
+                        err = -EIO;
+                        ll_rw_block(READ, 1, &bh);
+                        wait_on_buffer(bh);
+                        /* Uhhuh. Read error. Complain and punt.*/
+                        if (!buffer_uptodate(bh))
+                                goto next;
+                }
+
+                if (ext4_should_journal_data(inode)) {
+                        BUFFER_TRACE(bh, "get write access");
+                        err = ext4_journal_get_write_access(handle, bh);
+                        if (err)
+                                goto next;
+                }
+
+                zero_user(page, pos, range_to_discard);
+
+                err = 0;
+                if (ext4_should_journal_data(inode)) {
+                        err = ext4_handle_dirty_metadata(handle, inode, bh);
+                } else {
+                        if (ext4_should_order_data(inode) &&
+                           EXT4_I(inode)->jinode)
+                                err = ext4_jbd2_file_inode(handle, inode);
+                        mark_buffer_dirty(bh);
+                }
+
+                BUFFER_TRACE(bh, "Partial buffer zeroed");
+next:
+                bh = bh->b_this_page;
+                iblock++;
+                pos += range_to_discard;
+        }
+
+        return err;
+}
+
 /*
  * ext4_block_truncate_page() zeroes out a mapping from file offset `from'
  * up to the end of the block which corresponds to `from'.