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Date:   Mon, 13 Jan 2020 19:41:37 -0500
From:   "Theodore Y. Ts'o" <>
To:     Eric Biggers <>
Cc:,,, Victor Hsieh <>,
Subject: Re: [PATCH v3] fs-verity: implement readahead for

On Mon, Jan 06, 2020 at 12:54:10PM -0800, Eric Biggers wrote:
> From: Eric Biggers <>
> 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 <>

Looks good, feel free to add:

Reviewed-by: Theodore Ts'o <>

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