linux-ext4 - Re: [PATCH v2 21/23] xfs: handle merkle tree block size != fs blocksize != PAGE

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Date:   Tue, 4 Apr 2023 16:32:24 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Andrey Albershteyn <aalbersh@...hat.com>
Cc:     djwong@...nel.org, dchinner@...hat.com, hch@...radead.org,
        linux-xfs@...r.kernel.org, fsverity@...ts.linux.dev,
        rpeterso@...hat.com, agruenba@...hat.com, xiang@...nel.org,
        chao@...nel.org, damien.lemoal@...nsource.wdc.com, jth@...nel.org,
        linux-erofs@...ts.ozlabs.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com
Subject: Re: [PATCH v2 21/23] xfs: handle merkle tree block size != fs
 blocksize != PAGE_SIZE

Hi Andrey,

On Tue, Apr 04, 2023 at 04:53:17PM +0200, Andrey Albershteyn wrote:
> In case of different Merkle tree block size fs-verity expects
> ->read_merkle_tree_page() to return Merkle tree page filled with
> Merkle tree blocks. The XFS stores each merkle tree block under
> extended attribute. Those attributes are addressed by block offset
> into Merkle tree.
> 
> This patch make ->read_merkle_tree_page() to fetch multiple merkle
> tree blocks based on size ratio. Also the reference to each xfs_buf
> is passed with page->private to ->drop_page().
> 
> Signed-off-by: Andrey Albershteyn <aalbersh@...hat.com>
> ---
>  fs/xfs/xfs_verity.c | 74 +++++++++++++++++++++++++++++++++++----------
>  fs/xfs/xfs_verity.h |  8 +++++
>  2 files changed, 66 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/xfs/xfs_verity.c b/fs/xfs/xfs_verity.c
> index a9874ff4efcd..ef0aff216f06 100644
> --- a/fs/xfs/xfs_verity.c
> +++ b/fs/xfs/xfs_verity.c
> @@ -134,6 +134,10 @@ xfs_read_merkle_tree_page(
>  	struct page		*page = NULL;
>  	__be64			name = cpu_to_be64(index << PAGE_SHIFT);
>  	uint32_t		bs = 1 << log_blocksize;
> +	int			blocks_per_page =
> +		(1 << (PAGE_SHIFT - log_blocksize));
> +	int			n = 0;
> +	int			offset = 0;
>  	struct xfs_da_args	args = {
>  		.dp		= ip,
>  		.attr_filter	= XFS_ATTR_VERITY,
> @@ -143,26 +147,59 @@ xfs_read_merkle_tree_page(
>  		.valuelen	= bs,
>  	};
>  	int			error = 0;
> +	bool			is_checked = true;
> +	struct xfs_verity_buf_list	*buf_list;
>  
>  	page = alloc_page(GFP_KERNEL);
>  	if (!page)
>  		return ERR_PTR(-ENOMEM);
>  
> -	error = xfs_attr_get(&args);
> -	if (error) {
> -		kmem_free(args.value);
> -		xfs_buf_rele(args.bp);
> +	buf_list = kzalloc(sizeof(struct xfs_verity_buf_list), GFP_KERNEL);
> +	if (!buf_list) {
>  		put_page(page);
> -		return ERR_PTR(-EFAULT);
> +		return ERR_PTR(-ENOMEM);
>  	}
>  
> -	if (args.bp->b_flags & XBF_VERITY_CHECKED)
> +	/*
> +	 * Fill the page with Merkle tree blocks. The blcoks_per_page is higher
> +	 * than 1 when fs block size != PAGE_SIZE or Merkle tree block size !=
> +	 * PAGE SIZE
> +	 */
> +	for (n = 0; n < blocks_per_page; n++) {
> +		offset = bs * n;
> +		name = cpu_to_be64(((index << PAGE_SHIFT) + offset));
> +		args.name = (const uint8_t *)&name;
> +
> +		error = xfs_attr_get(&args);
> +		if (error) {
> +			kmem_free(args.value);
> +			/*
> +			 * No more Merkle tree blocks (e.g. this was the last
> +			 * block of the tree)
> +			 */
> +			if (error == -ENOATTR)
> +				break;
> +			xfs_buf_rele(args.bp);
> +			put_page(page);
> +			kmem_free(buf_list);
> +			return ERR_PTR(-EFAULT);
> +		}
> +
> +		buf_list->bufs[buf_list->buf_count++] = args.bp;
> +
> +		/* One of the buffers was dropped */
> +		if (!(args.bp->b_flags & XBF_VERITY_CHECKED))
> +			is_checked = false;
> +
> +		memcpy(page_address(page) + offset, args.value, args.valuelen);
> +		kmem_free(args.value);
> +		args.value = NULL;
> +	}

I was really hoping for a solution where the cached data can be used directly,
instead allocating a temporary page and copying the cached data into it every
time the cache is accessed.  The problem with what you have now is that every
time a single 32-byte hash is accessed, a full page (potentially 64KB!) will be
allocated and filled.  That's not very efficient.  The need to allocate the
temporary page can also cause ENOMEM (which will get reported as EIO).

Did you consider alternatives that would work more efficiently?  I think it
would be worth designing something that works properly with how XFS is planned
to cache the Merkle tree, instead of designing a workaround.
->read_merkle_tree_page was not really designed for what you are doing here.

How about replacing ->read_merkle_tree_page with a function that takes in a
Merkle tree block index (not a page index!) and hands back a (page, offset) pair
that identifies where the Merkle tree block's data is located?  Or (folio,
offset), I suppose.

With that, would it be possible to directly return the XFS cache?

- Eric