Re: [PATCH v2] fs-verity: implement readahead for FS_IOC_ENABLE_VERITY

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Dec 10, 2019 at 10:35:31AM -0800, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> When it builds the first level of the Merkle tree, FS_IOC_ENABLE_VERITY
> sequentially reads each page of the file using read_mapping_page().
> This works fine if the file's data is already in pagecache, which should
> normally be the case, since this ioctl is normally used immediately
> after writing out the file.
> 
> But in any other case this implementation performs very poorly, since
> only one page is read at a time.
> 
> Fix this by implementing readahead using the functions from
> mm/readahead.c.
> 
> This improves performance in the uncached case by about 20x, as seen in
> the following benchmarks done on a 250MB file (on x86_64 with SHA-NI):
> 
>     FS_IOC_ENABLE_VERITY uncached (before) 3.299s
>     FS_IOC_ENABLE_VERITY uncached (after)  0.160s
>     FS_IOC_ENABLE_VERITY cached            0.147s
>     sha256sum uncached                     0.191s
>     sha256sum cached                       0.145s
> 
> Note: we could instead switch to kernel_read().  But that would mean
> we'd no longer be hashing the data directly from the pagecache, which is
> a nice optimization of its own.  And using kernel_read() would require
> allocating another temporary buffer, hashing the data and tree pages
> separately, and explicitly zero-padding the last page -- so it wouldn't
> really be any simpler than direct pagecache access, at least for now.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> 
> Changed v1 => v2:
> - Only do sync readahead when the page wasn't found in the pagecache at all.
> - Use ->f_mapping so that the inode doesn't have to be passed.
> 
> 
>  fs/verity/enable.c | 45 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 39 insertions(+), 6 deletions(-)
> 

Applied to fscrypt.git#fsverity for 5.6.

- Eric