linux-kernel - Re: [PATCH RFC] introduce ioctl to completely invalidate page cache

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Message-ID: <20141007191624.GD30038@quack.suse.cz>
Date:	Tue, 7 Oct 2014 21:16:24 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>,
	Thanos Makatos <thanos.makatos@...rix.com>,
	Jens Axboe <axboe@...nel.dk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
	"jlayton@...chiereds.net" <jlayton@...chiereds.net>,
	"bfields@...ldses.org" <bfields@...ldses.org>
Subject: Re: [PATCH RFC] introduce ioctl to completely invalidate page cache

On Tue 07-10-14 12:30:59, Dave Chinner wrote:
> On Mon, Oct 06, 2014 at 04:30:19PM +0200, Jan Kara wrote:
> > On Mon 06-10-14 11:33:23, Thanos Makatos wrote:
> > > > > Trond also had a comment that if we extended the ioctl to work for all
> > > > > inodes (not just blkdev) and allowed some additional flags of what
> > > > > needs to be invalidated, the new ioctl would be also useful to NFS
> > > > > userspace - see Trond's email at
> > > > >
> > > > > http://www.spinics.net/lists/linux-fsdevel/msg78917.html
> > > > >
> > > > > and the following thread. I would prefer to cover that usecase when we
> > > > > are introducing new invalidation ioctl. Have you considered that Thanos?
> > > > 
> > > > Sure, though I don't really know how to do it. I'll start by looking at the code
> > > > flow when someone does " echo 3 > /proc/sys/vm/drop_caches", unless you
> > > > already have a rough idea how to do that.
> > > 
> > > I realise I haven't clearly understood what the semantics of this new ioctl
> > > should be.
> > > 
> > > My initial goal was to implement an ioctl that would _completely_ invalidate
> > > the buffer cache of a block device when there is no file-system involved.
> > > Unless I'm mistaken the patch I posted achieves this goal.
> >   Yes.
> > 
> > > We now want to extend this patch to take care of cached metadata, which seems
> > > to be of particular importance for NFS, and I suspect that this piece of
> > > functionality will still be applicable to any kind of file-system, correct?
> >   So most notably they want the ioctl to work not only for block devices
> > but also for any regular file. That's easily doable - you just call
> > filemap_write_and_wait() and invalidate_inode_pages2() in the ioctl handler
> > for regular files.
> > 
> > Also they wanted to be able to specify a range of a mapping to invalidate -
> > that's easily doable as well. Finally they wanted a 'flags' argument so you
> > can additionally ask fs to invalidate also some metadata. How invalidation
> > is done will be a fs specific thing and for now I guess we don't need to go
> > into details. NFS guys can sort that out when they decide to implement it.
> > So in the beginning we can just have u64 flags argument and in
> > it a single 'INVAL_DATA' flag meaning that invalidation of data in a given
> > range is requested. Later NFS guys can add further flags.
> 
> Why do we need a new ioctl to do this? fadvise64() seems like it's
> the exact fit for "FADV_INVALIDATE_[META]DATA" flags...
  Well, fadvise() is currently a hint to kernel. In this case we would
really like the call to do the invalidation and return error if it fails
for some reason. So I'm not sure fadvise() is a perfect fit. But I wouldn't
be strongly opposed to it either.
								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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