Re: [PATCH v3] ext4: Rewrite ext4_page_mkwrite() to use generic helpers

[Eric Sandeen <sandeen@redhat.com>]

On 6/22/11 1:24 PM, Jan Kara wrote:
> Rewrite ext4_page_mkwrite() to use __block_page_mkwrite() helper. This
> removes the need of using i_alloc_sem to avoid races with truncate which
> seems to be the wrong locking order according to lock ordering documented in
> mm/rmap.c. Also calling ext4_da_write_begin() as used by the old code seems to
> be problematic because we can decide to flush delay-allocated blocks which
> will acquire s_umount semaphore - again creating unpleasant lock dependency
> if not directly a deadlock.

I have a customer testcase which reliably locks up with freeze & mmap, and
with this patch in place (and the other 2 higher-level patches that made
it into 3.0-rcX) it passes, FWIW.

-Eric

> Also add a check for frozen filesystem so that we don't busyloop in page fault
> when the filesystem is frozen.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/ext4/inode.c |  106 ++++++++++++++++++++++++++++--------------------------
>  1 files changed, 55 insertions(+), 51 deletions(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e3126c0..bd30976 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5843,80 +5843,84 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
>  	struct page *page = vmf->page;
>  	loff_t size;
>  	unsigned long len;
> -	int ret = -EINVAL;
> -	void *fsdata;
> +	int ret;
>  	struct file *file = vma->vm_file;
>  	struct inode *inode = file->f_path.dentry->d_inode;
>  	struct address_space *mapping = inode->i_mapping;
> +	handle_t *handle;
> +	get_block_t *get_block;
> +	int retries = 0;
>  
>  	/*
> -	 * Get i_alloc_sem to stop truncates messing with the inode. We cannot
> -	 * get i_mutex because we are already holding mmap_sem.
> +	 * This check is racy but catches the common case. We rely on
> +	 * __block_page_mkwrite() to do a reliable check.
>  	 */
> -	down_read(&inode->i_alloc_sem);
> -	size = i_size_read(inode);
> -	if (page->mapping != mapping || size <= page_offset(page)
> -	    || !PageUptodate(page)) {
> -		/* page got truncated from under us? */
> -		goto out_unlock;
> +	vfs_check_frozen(inode->i_sb, SB_FREEZE_WRITE);
> +	/* Delalloc case is easy... */
> +	if (test_opt(inode->i_sb, DELALLOC) &&
> +	    !ext4_should_journal_data(inode) &&
> +	    !ext4_nonda_switch(inode->i_sb)) {
> +		do {
> +			ret = __block_page_mkwrite(vma, vmf,
> +						   ext4_da_get_block_prep);
> +		} while (ret == -ENOSPC &&
> +		       ext4_should_retry_alloc(inode->i_sb, &retries));
> +		goto out_ret;
>  	}
> -	ret = 0;
>  
>  	lock_page(page);
> -	wait_on_page_writeback(page);
> -	if (PageMappedToDisk(page)) {
> -		up_read(&inode->i_alloc_sem);
> -		return VM_FAULT_LOCKED;
> +	size = i_size_read(inode);
> +	/* Page got truncated from under us? */
> +	if (page->mapping != mapping || page_offset(page) > size) {
> +		unlock_page(page);
> +		ret = VM_FAULT_NOPAGE;
> +		goto out;
>  	}
>  
>  	if (page->index == size >> PAGE_CACHE_SHIFT)
>  		len = size & ~PAGE_CACHE_MASK;
>  	else
>  		len = PAGE_CACHE_SIZE;
> -
>  	/*
> -	 * return if we have all the buffers mapped. This avoid
> -	 * the need to call write_begin/write_end which does a
> -	 * journal_start/journal_stop which can block and take
> -	 * long time
> +	 * Return if we have all the buffers mapped. This avoids the need to do
> +	 * journal_start/journal_stop which can block and take a long time
>  	 */
>  	if (page_has_buffers(page)) {
>  		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
>  					ext4_bh_unmapped)) {
> -			up_read(&inode->i_alloc_sem);
> -			return VM_FAULT_LOCKED;
> +			/* Wait so that we don't change page under IO */
> +			wait_on_page_writeback(page);
> +			ret = VM_FAULT_LOCKED;
> +			goto out;
>  		}
>  	}
>  	unlock_page(page);
> -	/*
> -	 * OK, we need to fill the hole... Do write_begin write_end
> -	 * to do block allocation/reservation.We are not holding
> -	 * inode.i__mutex here. That allow * parallel write_begin,
> -	 * write_end call. lock_page prevent this from happening
> -	 * on the same page though
> -	 */
> -	ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
> -			len, AOP_FLAG_UNINTERRUPTIBLE, &page, &fsdata);
> -	if (ret < 0)
> -		goto out_unlock;
> -	ret = mapping->a_ops->write_end(file, mapping, page_offset(page),
> -			len, len, page, fsdata);
> -	if (ret < 0)
> -		goto out_unlock;
> -	ret = 0;
> -
> -	/*
> -	 * write_begin/end might have created a dirty page and someone
> -	 * could wander in and start the IO.  Make sure that hasn't
> -	 * happened.
> -	 */
> -	lock_page(page);
> -	wait_on_page_writeback(page);
> -	up_read(&inode->i_alloc_sem);
> -	return VM_FAULT_LOCKED;
> -out_unlock:
> -	if (ret)
> +	/* OK, we need to fill the hole... */
> +	if (ext4_should_dioread_nolock(inode))
> +		get_block = ext4_get_block_write;
> +	else
> +		get_block = ext4_get_block;
> +retry_alloc:
> +	handle = ext4_journal_start(inode, ext4_writepage_trans_blocks(inode));
> +	if (IS_ERR(handle)) {
>  		ret = VM_FAULT_SIGBUS;
> -	up_read(&inode->i_alloc_sem);
> +		goto out;
> +	}
> +	ret = __block_page_mkwrite(vma, vmf, get_block);
> +	if (!ret && ext4_should_journal_data(inode)) {
> +		if (walk_page_buffers(handle, page_buffers(page), 0,
> +			  PAGE_CACHE_SIZE, NULL, do_journal_get_write_access)) {
> +			unlock_page(page);
> +			ret = VM_FAULT_SIGBUS;
> +			goto out;
> +		}
> +		ext4_set_inode_state(inode, EXT4_STATE_JDATA);
> +	}
> +	ext4_journal_stop(handle);
> +	if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> +		goto retry_alloc;
> +out_ret:
> +	ret = block_page_mkwrite_return(ret);
> +out:
>  	return ret;
>  }