linux-ext4 - Re: [PATCH 1/6] ext4: Fix races between page faults and hole punching

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Message-ID: <20151015091416.GB5234@quack.suse.cz>
Date:	Thu, 15 Oct 2015 11:14:16 +0200
From:	Jan Kara <jack@...e.cz>
To:	Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:	Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
	Dan Williams <dan.j.williams@...el.com>,
	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 1/6] ext4: Fix races between page faults and hole punching

On Wed 14-10-15 21:00:59, Ross Zwisler wrote:
> On Wed, Oct 14, 2015 at 01:30:22PM +0200, Jan Kara wrote:
> > Currently, page faults and hole punching are completely unsynchronized.
> > This can result in page fault faulting in a page into a range that we
> > are punching after truncate_pagecache_range() has been called and thus
> > we can end up with a page mapped to disk blocks that will be shortly
> > freed. Filesystem corruption will shortly follow. Note that the same
> > race is avoided for truncate by checking page fault offset against
> > i_size but there isn't similar mechanism available for punching holes.
> > 
> > Fix the problem by creating new rw semaphore i_mmap_sem in inode and
> > grab it for writing over truncate and hole punching and for read over
> > page faults. We cannot easily use i_data_sem for this since that ranks
> > below transaction start and we need something ranking above it so that
> > it can be held over the whole truncate / hole punching operation.
> > 
> > Signed-off-by: Jan Kara <jack@...e.com>
> > ---
> >  fs/ext4/ext4.h  | 10 +++++++++
> >  fs/ext4/file.c  | 66 +++++++++++++++++++++++++++++++++++++++++++++++++--------
> >  fs/ext4/inode.c | 27 +++++++++++++++++++----
> >  fs/ext4/super.c |  1 +
> >  4 files changed, 91 insertions(+), 13 deletions(-)
> 
> I wonder if there are a few other operations in ext4_fallocate() that
> we may need to protect in addition to ext4_punch_hole()?
> 
> Do ext4_collapse_range(), ext4_insert_range() and maybe even
> ext4_zero_range() need protection?

Yeah, they do. I'll recheck what have I missed... Thanks for catching this!

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