Re: [RFCv3 4/7] ext2: Implement seq counter for validating cached iomap

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Apr 25, 2024 at 06:58:48PM +0530, Ritesh Harjani (IBM) wrote:
> There is a possibility of following race with iomap during
> writebck -
> 
> write_cache_pages()
>   cache extent covering 0..1MB range
>   write page at offset 0k
> 					truncate(file, 4k)
> 					  drops all relevant pages
> 					  frees fs blocks
> 					pwrite(file, 4k, 4k)
> 					  creates dirty page in the page cache
>   writes page at offset 4k to a stale block
> 
> This race can happen because iomap_writepages() keeps a cached extent mapping
> within struct iomap. While write_cache_pages() is going over each folio,
> (can cache a large extent range), if a truncate happens in parallel on the
> next folio followed by a buffered write to the same offset within the file,
> this can change logical to physical offset of the cached iomap mapping.
> That means, the cached iomap has now become stale.
> 
> This patch implements the seq counter approach for revalidation of stale
> iomap mappings. i_blkseq will get incremented for every block
> allocation/free. Here is what we do -
> 
> For ext2 buffered-writes, the block allocation happens at the
> ->write_iter time itself. So at writeback time,
> 1. We first cache the i_blkseq.
> 2. Call ext2_get_blocks(, create = 0) to get the no. of blocks
>    already allocated.
> 3. Call ext2_get_blocks() the second time with length to be same as
>    the no. of blocks we know were already allocated.
> 4. Till now it means, the cached i_blkseq remains valid as no block
>    allocation has happened yet.
> This means the next call to ->map_blocks(), we can verify whether the
> i_blkseq has raced with truncate or not. If not, then i_blkseq will
> remain valid.
> 
> In case of a hole (could happen with mmaped writes), we only allocate
> 1 block at a time anyways. So even if the i_blkseq value changes right
> after, we anyway need to allocate the next block in subsequent
> ->map_blocks() call.
> 
> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
> ---
>  fs/ext2/balloc.c |  1 +
>  fs/ext2/ext2.h   |  6 +++++
>  fs/ext2/inode.c  | 57 ++++++++++++++++++++++++++++++++++++++++++++----
>  fs/ext2/super.c  |  2 +-
>  4 files changed, 61 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext2/balloc.c b/fs/ext2/balloc.c
> index 1bfd6ab11038..047a8f41a6f5 100644
> --- a/fs/ext2/balloc.c
> +++ b/fs/ext2/balloc.c
> @@ -495,6 +495,7 @@ void ext2_free_blocks(struct inode * inode, ext2_fsblk_t block,
>  	}
>  
>  	ext2_debug ("freeing block(s) %lu-%lu\n", block, block + count - 1);
> +	ext2_inc_i_blkseq(EXT2_I(inode));
>  
>  do_more:
>  	overflow = 0;
> diff --git a/fs/ext2/ext2.h b/fs/ext2/ext2.h
> index f38bdd46e4f7..67b1acb08eb2 100644
> --- a/fs/ext2/ext2.h
> +++ b/fs/ext2/ext2.h
> @@ -663,6 +663,7 @@ struct ext2_inode_info {
>  	struct rw_semaphore xattr_sem;
>  #endif
>  	rwlock_t i_meta_lock;
> +	unsigned int i_blkseq;
>  
>  	/*
>  	 * truncate_mutex is for serialising ext2_truncate() against
> @@ -698,6 +699,11 @@ static inline struct ext2_inode_info *EXT2_I(struct inode *inode)
>  	return container_of(inode, struct ext2_inode_info, vfs_inode);
>  }
>  
> +static inline void ext2_inc_i_blkseq(struct ext2_inode_info *ei)
> +{
> +	WRITE_ONCE(ei->i_blkseq, READ_ONCE(ei->i_blkseq) + 1);
> +}
> +
>  /* balloc.c */
>  extern int ext2_bg_has_super(struct super_block *sb, int group);
>  extern unsigned long ext2_bg_num_gdb(struct super_block *sb, int group);
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 2b62786130b5..946a614ddfc0 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -406,6 +406,8 @@ static int ext2_alloc_blocks(struct inode *inode,
>  	ext2_fsblk_t current_block = 0;
>  	int ret = 0;
>  
> +	ext2_inc_i_blkseq(EXT2_I(inode));
> +
>  	/*
>  	 * Here we try to allocate the requested multiple blocks at once,
>  	 * on a best-effort basis.
> @@ -966,15 +968,62 @@ ext2_writepages(struct address_space *mapping, struct writeback_control *wbc)
>  	return mpage_writepages(mapping, wbc, ext2_get_block);
>  }
>  
> +static bool ext2_imap_valid(struct iomap_writepage_ctx *wpc, struct inode *inode,
> +			    loff_t offset)

ext2_iomap_valid, to stay consistent with the ext4 conversion series?

> +{
> +	if (offset < wpc->iomap.offset ||
> +	    offset >= wpc->iomap.offset + wpc->iomap.length)
> +		return false;
> +
> +	if (wpc->iomap.validity_cookie != READ_ONCE(EXT2_I(inode)->i_blkseq))
> +		return false;
> +
> +	return true;
> +}
> +
>  static int ext2_write_map_blocks(struct iomap_writepage_ctx *wpc,
>  				 struct inode *inode, loff_t offset,
>  				 unsigned len)
>  {
> -	if (offset >= wpc->iomap.offset &&
> -	    offset < wpc->iomap.offset + wpc->iomap.length)
> +	loff_t maxblocks = (loff_t)INT_MAX;
> +	u8 blkbits = inode->i_blkbits;
> +	u32 bno;
> +	bool new, boundary;
> +	int ret;
> +
> +	if (ext2_imap_valid(wpc, inode, offset))
>  		return 0;
>  
> -	return ext2_iomap_begin(inode, offset, inode->i_sb->s_blocksize,
> +	/*
> +	 * For ext2 buffered-writes, the block allocation happens at the
> +	 * ->write_iter time itself. So at writeback time -
> +	 * 1. We first cache the i_blkseq.
> +	 * 2. Call ext2_get_blocks(, create = 0) to get the no. of blocks
> +	 *    already allocated.
> +	 * 3. Call ext2_get_blocks() the second time with length to be same as
> +	 *    the no. of blocks we know were already allocated.
> +	 * 4. Till now it means, the cached i_blkseq remains valid as no block
> +	 *    allocation has happened yet.
> +	 * This means the next call to ->map_blocks(), we can verify whether the
> +	 * i_blkseq has raced with truncate or not. If not, then i_blkseq will
> +	 * remain valid.
> +	 *
> +	 * In case of a hole (could happen with mmaped writes), we only allocate
> +	 * 1 block at a time anyways. So even if the i_blkseq value changes, we
> +	 * anyway need to allocate the next block in subsequent ->map_blocks()
> +	 * call.

You might want to leave a comment here that ext2 doesn't support
unwritten extents, so the validation cookie is needed only for
writeback, and not for pagecache writes themselves.  I don't expect
anyone to port extents (and hence unwritten blocks) to ext2, so this is
a minor point.

> +	 */
> +	wpc->iomap.validity_cookie = READ_ONCE(EXT2_I(inode)->i_blkseq);
> +
> +	ret = ext2_get_blocks(inode, offset >> blkbits, maxblocks << blkbits,
> +			      &bno, &new, &boundary, 0);
> +	if (ret < 0)
> +		return ret;
> +	/*
> +	 * ret can be 0 in case of a hole which is possible for mmaped writes.
> +	 */
> +	ret = ret ? ret : 1;
> +	return ext2_iomap_begin(inode, offset, (loff_t)ret << blkbits,
>  				IOMAP_WRITE, &wpc->iomap, NULL);
>  }
>  
> @@ -1000,7 +1049,7 @@ ext2_dax_writepages(struct address_space *mapping, struct writeback_control *wbc
>  
>  const struct address_space_operations ext2_file_aops = {
>  	.dirty_folio		= iomap_dirty_folio,
> -	.release_folio 		= iomap_release_folio,
> +	.release_folio		= iomap_release_folio,

This fix should be in patch 2.

--D

>  	.invalidate_folio	= iomap_invalidate_folio,
>  	.read_folio		= ext2_file_read_folio,
>  	.readahead		= ext2_file_readahead,
> diff --git a/fs/ext2/super.c b/fs/ext2/super.c
> index 37f7ce56adce..32f5386284d6 100644
> --- a/fs/ext2/super.c
> +++ b/fs/ext2/super.c
> @@ -188,7 +188,7 @@ static struct inode *ext2_alloc_inode(struct super_block *sb)
>  #ifdef CONFIG_QUOTA
>  	memset(&ei->i_dquot, 0, sizeof(ei->i_dquot));
>  #endif
> -
> +	WRITE_ONCE(ei->i_blkseq, 0);
>  	return &ei->vfs_inode;
>  }
>  
> -- 
> 2.44.0
> 
>