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

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Date:   Tue, 4 Jun 2019 10:00:43 +0200
From:   Jan Kara <jack@...e.cz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jan Kara <jack@...e.cz>, 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 03-06-19 19:16:59, Amir Goldstein wrote:
> 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

Yes, I've noticed this.

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

Ah, that's an interesting idea. I'll try that out. It will require some
dance in madvise() to drop mmap_sem but we already do that for
madvise_free() so I can just duplicate that.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR