David Chinner wrote:
Generic page_mkwrite functionality.
Filesystems that make use of the VM ->page_mkwrite() callout will generally use
the same core code to implement it. There are several tricky truncate-related
issues that we need to deal with here as we cannot take the i_mutex as we
normally would for these paths. These issues are not documented anywhere yet
so block_page_mkwrite() seems like the best place to start.
Version 2:
- read inode size only once
- more comments explaining implementation restrictions
Signed-Off-By: Dave Chinner <dgc@xxxxxxx>
---
fs/buffer.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
include/linux/buffer_head.h | 2 +
2 files changed, 49 insertions(+)
Index: 2.6.x-xfs-new/fs/buffer.c
===================================================================
--- 2.6.x-xfs-new.orig/fs/buffer.c 2007-03-17 10:55:32.291414968 +1100
+++ 2.6.x-xfs-new/fs/buffer.c 2007-03-19 08:13:54.519909087 +1100
@@ -2194,6 +2194,52 @@ int generic_commit_write(struct file *fi
return 0;
}
+/*
+ * block_page_mkwrite() is not allowed to change the file size as it gets
+ * called from a page fault handler when a page is first dirtied. Hence we must
+ * be careful to check for EOF conditions here. We set the page up correctly
+ * for a written page which means we get ENOSPC checking when writing into
+ * holes and correct delalloc and unwritten extent mapping on filesystems that
+ * support these features.
+ *
+ * We are not allowed to take the i_mutex here so we have to play games to
+ * protect against truncate races as the page could now be beyond EOF. Because
+ * vmtruncate() writes the inode size before removing pages, once we have the
+ * page lock we can determine safely if the page is beyond EOF. If it is not
+ * beyond EOF, then the page is guaranteed safe against truncation until we
+ * unlock the page.
+ */
+int
+block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
+ get_block_t get_block)
+{
+ struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
+ unsigned long end;
+ loff_t size;
+ int ret = -EINVAL;
+
+ lock_page(page);
+ size = i_size_read(inode);
+ if ((page->mapping != inode->i_mapping) ||
+ ((page->index << PAGE_CACHE_SHIFT) > size)) {
+ /* page got truncated out from underneath us */
+ goto out_unlock;
+ }
I see your explanation above, but I still don't see why this can't
just follow the conventional if (!page->mapping) check for truncation.
If the test happens to be performed after truncate concurrently
decreases i_size, then the blocks are going to get truncated by the
truncate afterwards anyway.
+
+ /* page is wholly or partially inside EOF */
+ if (((page->index + 1) << PAGE_CACHE_SHIFT) > size)
+ end = size & ~PAGE_CACHE_MASK;
+ else
+ end = PAGE_CACHE_SIZE;
+
+ ret = block_prepare_write(page, 0, end, get_block);
+ if (!ret)
+ ret = block_commit_write(page, 0, end);
+
+out_unlock:
+ unlock_page(page);
+ return ret;
+}
--
SUSE Labs, Novell Inc.
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