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

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Message-ID: <2368A3FCF9F7214298E53C823B0A48EC0424106C@AMSPEX01CL02.citrite.net>
Date:	Mon, 6 Oct 2014 11:33:23 +0000
From:	Thanos Makatos <thanos.makatos@...rix.com>
To:	Thanos Makatos <thanos.makatos@...rix.com>,
	'Jan Kara' <jack@...e.cz>, Jens Axboe <axboe@...nel.dk>
CC:	"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

> > 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.

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?

Do we want this new ioctl to do what "echo 3 > /proc/sys/vm/drop_caches" does
but on a more selective basis, which IIUC drops whatever can be dropped, but
may not drop everything? If so, then we should more precisely define this ioctl
as "drop *all* cached data and as much metadata you can, which may not be all
of them". If this is the case, would adding a call to drop_pagecache_sb() on
all super blocks whose .s_bdev equals the block device we're interested in
suffice?
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