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On Wed, Mar 18, 2009 at 02:34:25AM -0400, Christoph Hellwig wrote:
> As pointed out by Dave in the thread starting at
> http://oss.sgi.com/archives/xfs/2008-04/msg00542.html the current use
> of vmtruncate in block_write_begin is incorrect for filesystem not
> using ->truncate for doing the actual on-disk truncatation. Doing
> it in ->truncate for a filesystem like XFS means it would be split
> over multiple transactions leading to violations of the atomicy guarantee.
> Historically (and still documented in Documentation/filesystems/Locking)
> ->truncate is not a method but only a helper for setattr implementations
> so this is correct.
>
> The correct fix would be to use ->setattr but that needs a new ATTR_NOLOCK
> flag and an audit of all filesystems. I'm still hoping to do that later
> but for now I really want XFS fixed.
>
> So just duplicate block_write_begin in XFS to do the proper truncation,
> note that I need to export __block_prepare_write to no duplicate even
> more code than nessecary.
>
> GFS2 and UFS and possibly others wants the same kind of fix, too.
>
> And I need to get rid of ->truncate one day to prevent people from using
> it in stupid ways.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
>
> Index: xfs/fs/buffer.c
> ===================================================================
> --- xfs.orig/fs/buffer.c 2009-03-01 04:22:47.091430206 +0100
> +++ xfs/fs/buffer.c 2009-03-01 04:22:59.913336991 +0100
> @@ -1910,7 +1910,7 @@ void page_zero_new_buffers(struct page *
> }
> EXPORT_SYMBOL(page_zero_new_buffers);
>
> -static int __block_prepare_write(struct inode *inode, struct page *page,
> +int __block_prepare_write(struct inode *inode, struct page *page,
> unsigned from, unsigned to, get_block_t *get_block)
> {
> unsigned block_start, block_end;
> @@ -1989,6 +1989,7 @@ static int __block_prepare_write(struct
> page_zero_new_buffers(page, from, to);
> return err;
> }
> +EXPORT_SYMBOL(__block_prepare_write);
>
> static int __block_commit_write(struct inode *inode, struct page *page,
> unsigned from, unsigned to)
> Index: xfs/fs/xfs/linux-2.6/xfs_aops.c
> ===================================================================
> --- xfs.orig/fs/xfs/linux-2.6/xfs_aops.c 2009-03-01 04:16:12.003305119
> +0100
> +++ xfs/fs/xfs/linux-2.6/xfs_aops.c 2009-03-01 04:21:55.618306704 +0100
> @@ -1598,6 +1598,71 @@ xfs_vm_direct_IO(
> return ret;
> }
>
> +/*
> + * This is a copy of fs/buffer.c:block_write_begin except for doing
> + * the correct setattr call in the error case instead of the wrong
> + * vmtruncate.
> + */
> +static int
> +xfs_block_write_begin(struct file *file, struct address_space *mapping,
> + loff_t pos, unsigned len, unsigned flags,
> + struct page **pagep, void **fsdata,
> + get_block_t *get_block)
> +{
> + struct inode *inode = mapping->host;
> + int status = 0;
> + struct page *page;
> + pgoff_t index;
> + unsigned start, end;
> + int ownpage = 0;
> +
> + index = pos >> PAGE_CACHE_SHIFT;
> + start = pos & (PAGE_CACHE_SIZE - 1);
> + end = start + len;
> +
> + page = *pagep;
> + if (page == NULL) {
> + ownpage = 1;
> + page = grab_cache_page_write_begin(mapping, index, flags);
> + if (!page) {
> + status = -ENOMEM;
> + goto out;
> + }
> + *pagep = page;
> + } else
> + BUG_ON(!PageLocked(page));
> +
> + status = __block_prepare_write(inode, page, start, end, get_block);
> + if (unlikely(status)) {
> + ClearPageUptodate(page);
> +
> + if (ownpage) {
> + unlock_page(page);
> + page_cache_release(page);
> + *pagep = NULL;
> +
> + /*
> + * prepare_write() may have instantiated a few blocks
> + * outside i_size. Trim these off again. Don't need
> + * i_size_read because we hold i_mutex.
> + */
> + if (pos + len > inode->i_size) {
> + struct iattr newattrs;
> + int error;
> +
> + newattrs.ia_size = inode->i_size;
> + newattrs.ia_valid = ATTR_SIZE | ATTR_FORCE;
> + error = xfs_setattr(XFS_I(inode), &newattrs,
> + XFS_ATTR_NOLOCK);
> + WARN_ON(error); /* not much we can do.. */
> + }
> + }
> + }
> +
> +out:
> + return status;
> +}
> +
> STATIC int
> xfs_vm_write_begin(
> struct file *file,
> Index: xfs/include/linux/buffer_head.h
> ===================================================================
> --- xfs.orig/include/linux/buffer_head.h 2009-03-01 04:23:14.364305635
> +0100
> +++ xfs/include/linux/buffer_head.h 2009-03-01 04:23:19.081305583 +0100
> @@ -218,6 +218,8 @@ int generic_write_end(struct file *, str
> struct page *, void *);
> void page_zero_new_buffers(struct page *page, unsigned from, unsigned to);
> int block_prepare_write(struct page*, unsigned, unsigned, get_block_t*);
> +int __block_prepare_write(struct inode *inode, struct page *page,
> + unsigned from, unsigned to, get_block_t *get_block);
> int cont_write_begin(struct file *, struct address_space *, loff_t,
> unsigned, unsigned, struct page **, void **,
> get_block_t *, loff_t *);
>
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