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

[Dave Chinner <david@fromorbit.com>]

On Wed, Oct 14, 2015 at 09:00:59PM -0600, 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@suse.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?

Yes, they do. Anything that does direct extent manipulation needs
to invalidate current mappings across the range and that requires
page fault serialisation.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com