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

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Date:	Mon, 6 Oct 2014 09:21:17 +0000
From:	Thanos Makatos <thanos.makatos@...rix.com>
To:	'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

> > We're currently ignoring the buffer cache sync and invalidation (which
> > is odd), but at least being consistent would be good.
>   Well, invalidate_bdev() doesn't return anything. And
> invalidate_mapping_pages() inside invalidate_bdev() returns only number of
> invalidated pages. I don't think there's any value in returning that.
> 
> OTOH invalidate_inode_pages2() returns 0 / -EBUSY / other error when
> invalidation of some page fails so returning that seems useful.
> 
> > Might also need a filemap_write_and_wait() to sync before invalidation.
>   That's what fsync_bdev() is doing under the hoods. Sometimes I'm not sure
> whether all these wrappers are useful...

Indeed, fsync_bdev() does call filemap_write_and_wait() so I don't need to explicitly do that.

> 
> 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.

Thanks
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