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Date:   Wed, 5 Apr 2023 17:12:34 +0200
From:   Andrey Albershteyn <aalbersh@...hat.com>
To:     Eric Biggers <ebiggers@...nel.org>
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 Eric,

On Tue, Apr 04, 2023 at 04:32:24PM -0700, Eric Biggers wrote:
> 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
> 

Yeah, I also don't like it, I didn't want to change fs-verity much
so went with this workaround. But as it's ok, I will look into trying
to pass xfs buffers to fs-verity without direct use of
->read_merkle_tree_page(). I think it's possible with (folio,
offset), the xfs buffers aren't xattr value align so the 4k merkle
tree block is stored in two pages.

Thanks for suggestion!

-- 
- Andrey

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