Re: [PATCH] fs-verity: implement readahead of Merkle tree pages

[Eric Biggers <ebiggers@kernel.org>]

On Mon, Dec 16, 2019 at 10:11:12AM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> When fs-verity verifies data pages, currently it reads each Merkle tree
> page synchronously using read_mapping_page().
> 
> Therefore, when the Merkle tree pages aren't already cached, fs-verity
> causes an extra 4 KiB I/O request for every 512 KiB of data (assuming
> that the Merkle tree uses SHA-256 and 4 KiB blocks).  This results in
> more I/O requests and performance loss than is strictly necessary.
> 
> Therefore, implement readahead of the Merkle tree pages.
> 
> For simplicity, we take advantage of the fact that the kernel already
> does readahead of the file's *data*, just like it does for any other
> file.  Due to this, we don't really need a separate readahead state
> (struct file_ra_state) just for the Merkle tree, but rather we just need
> to piggy-back on the existing data readahead requests.
> 
> We also only really need to bother with the first level of the Merkle
> tree, since the usual fan-out factor is 128, so normally over 99% of
> Merkle tree I/O requests are for the first level.
> 
> Therefore, make fsverity_verify_bio() enable readahead of the first
> Merkle tree level, for up to 1/4 the number of pages in the bio, when it
> sees that the REQ_RAHEAD flag is set on the bio.  The readahead size is
> then passed down to ->read_merkle_tree_page() for the filesystem to
> (optionally) implement if it sees that the requested page is uncached.
> 
> While we're at it, also make build_merkle_tree_level() set the Merkle
> tree readahead size, since it's easy to do there.
> 
> However, for now don't set the readahead size in fsverity_verify_page(),
> since currently it's only used to verify holes on ext4 and f2fs, and it
> would need parameters added to know how much to read ahead.
> 
> This patch significantly improves fs-verity sequential read performance.
> Some quick benchmarks with 'cat'-ing a 250MB file after dropping caches:
> 
>     On ARM64 phone (using sha256-ce):
>         Before: 217 MB/s
>         After: 263 MB/s
>         (compare to sha256sum of non-verity file: 357 MB/s)
> 
>     In an x86_64 VM (using sha256-avx2):
>         Before: 173 MB/s
>         After: 215 MB/s
>         (compare to sha256sum of non-verity file: 223 MB/s)
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/ext4/verity.c             | 49 ++++++++++++++++++++++++++++++++++--
>  fs/f2fs/data.c               |  6 ++---
>  fs/f2fs/f2fs.h               |  3 +++
>  fs/f2fs/verity.c             | 49 ++++++++++++++++++++++++++++++++++--
>  fs/verity/enable.c           |  8 +++++-
>  fs/verity/fsverity_private.h |  1 +
>  fs/verity/open.c             |  1 +
>  fs/verity/verify.c           | 34 ++++++++++++++++++++-----
>  include/linux/fsverity.h     |  7 +++++-
>  9 files changed, 143 insertions(+), 15 deletions(-)

Ted and Jaegeuk, have you had a chance to review this patch?  I could use your
Acked-bys on it, since it touches fs/ext4/ and fs/f2fs/.

> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index a034cd0ce0217..8a6b3266bd794 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -1881,9 +1881,9 @@ static int f2fs_read_single_page(struct inode *inode, struct page *page,
>   * use ->readpage() or do the necessary surgery to decouple ->readpages()
>   * from read-ahead.
>   */
> -static int f2fs_mpage_readpages(struct address_space *mapping,
> -			struct list_head *pages, struct page *page,
> -			unsigned nr_pages, bool is_readahead)
> +int f2fs_mpage_readpages(struct address_space *mapping,
> +			 struct list_head *pages, struct page *page,
> +			 unsigned int nr_pages, bool is_readahead)
>  {

FYI, I'm aware that the f2fs compression patch (which is queued in f2fs/dev)
also makes f2fs_mpage_readpages() non-static, but uses slightly different
formatting.  If/when I apply this patch I'll adjust it to match f2fs/dev so that
there's no merge conflict.

- Eric