linux-ext4 - Re: [PATCH 1/2] mm: Add readahead file operation

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Date:   Mon, 3 Jun 2019 19:16:59 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Ext4 <linux-ext4@...r.kernel.org>, Ted Tso <tytso@....edu>,
        Linux MM <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH 1/2] mm: Add readahead file operation

On Mon, Jun 3, 2019 at 4:22 PM Jan Kara <jack@...e.cz> wrote:
>
> Some filesystems need to acquire locks before pages are read into page
> cache to protect from races with hole punching. The lock generally
> cannot be acquired within readpage as it ranks above page lock so we are
> left with acquiring the lock within filesystem's ->read_iter
> implementation for normal reads and ->fault implementation during page
> faults. That however does not cover all paths how pages can be
> instantiated within page cache - namely explicitely requested readahead.
> Add new ->readahead file operation which filesystem can use for this.
>
> CC: stable@...r.kernel.org # Needed by following ext4 fix
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  include/linux/fs.h |  5 +++++
>  include/linux/mm.h |  3 ---
>  mm/fadvise.c       | 12 +-----------
>  mm/madvise.c       |  3 ++-
>  mm/readahead.c     | 26 ++++++++++++++++++++++++--
>  5 files changed, 32 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index f7fdfe93e25d..9968abcd06ea 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1828,6 +1828,7 @@ struct file_operations {
>                                    struct file *file_out, loff_t pos_out,
>                                    loff_t len, unsigned int remap_flags);
>         int (*fadvise)(struct file *, loff_t, loff_t, int);
> +       int (*readahead)(struct file *, loff_t, loff_t);

The new method is redundant, because it is a subset of fadvise.
When overlayfs needed to implement both methods, Miklos
suggested that we unite them into one, hence:
3d8f7615319b vfs: implement readahead(2) using POSIX_FADV_WILLNEED

So you can accomplish the ext4 fix without the new method.
All you need extra is implementing madvise_willneed() with vfs_fadvise().

Thanks,
Amir.