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SubjectRe: [RFC] block integrity: Fix write after checksum calculation problem
On Fri, Mar 04, 2011 at 01:07:24PM -0800, Darrick J. Wong wrote:
> On Mon, Feb 28, 2011 at 07:54:05AM -0500, Chris Mason wrote:
> > Excerpts from Darrick J. Wong's message of 2011-02-24 13:27:32 -0500:
> > > On Thu, Feb 24, 2011 at 12:37:53PM -0500, Chris Mason wrote:
> > > > Excerpts from Jan Kara's message of 2011-02-24 11:47:58 -0500:
> > > > > On Wed 23-02-11 15:35:11, Chris Mason wrote:
> > > > > > Excerpts from Joel Becker's message of 2011-02-23 15:24:47 -0500:
> > > > > > > On Tue, Feb 22, 2011 at 11:45:44AM -0500, Martin K. Petersen wrote:
> > > > > > > > Also, DIX is only the tip of the iceberg. Many other impending
> > > > > > > > technologies feature checksums and require pages to be stable during I/O
> > > > > > > > due to checksumming, encryption and so on.
> > > > > > > >
> > > > > > > > The VM is already trying to do the right thing. We just need the
> > > > > > > > relevant filesystems to catch up.
> > > > > > >
> > > > > > > ocfs2 handles stable metadata for its checksums when feeding
> > > > > > > things to the journal. If we're doing pagecache-based I/O, is the
> > > > > > > pagecache going to help here for data?
> > > > > >
> > > > > > Data is much easier than metadata. All you really need is to wait on
> > > > > > writeback in file_write, wait on writeback in page_mkwrite, and make
> > > > > > sure you don't free blocks back to the allocator that are actively under
> > > > > > IO.
> > > > > >
> > > > > > I expect the hard part to be jbd and metadata in ext34.
> > > > > But JBD already has to do data copy if a buffer is going to be modified
> > > > > before/while it is written to the journal. So we should alredy do all that
> > > > > is needed for metadata. I don't say there aren't any bugs as they could be
> > > > > triggered only by crashing at the wrong moment and observing fs corruption.
> > > > > But most of the work should be there...
> > > >
> > > > Most of it is there, but there are always little bits and pieces. The
> > > > ext4 journal csumming code was one semi-recent example where we found
> > > > metadata changing in flight.
> > > >
> > > > A big part of testing this is getting some way to detect the bugs
> > > > without dif/dix. With btrfs I have patches to do set_memory_ro on
> > > > pages once I've don the crc, hopefully we can generalize that idea or
> > > > some up with something smarter.
> > >
> > > Right now I'm faking it with modprobe scsi_debug ato=1 guard=1 dif=3 dix=199.
> > >
> > > Hm, would you mind sharing those patches? I've been working on a second patch
> > > to do the wait-on-writeback per everyone's suggestions, but I still see the
> > > occasional corruption error as soon as I enable the mmap write case and covet
> > > some more debugging tools. It does seem to be working for the pure pwrite()
> > > case. :)
> >
> > Here's an ext4 version of the debugging patch. It's a few years old but
> > it'll give you the idea. This only covers metadata pages.
> >
> > Looks like I hacked the btrfs version up and didn't keep the original,
> > I'll have to rework it, I was trying to use it for the big corruption I
> > fixed recently and made a bunch of changes.
> >
> > For data if mmap is giving you trouble you need to wait on writeback in
> > page_mkwrite, with the page locked. fs/btrfs/inode.c has our
> > page_mkwrite, which uses wait_on_page_writeback() and also the btrfs
> > ordered write code. But for the other filesystems, waiting on writeback
> > should be enough.
>
> Ok, here's what I have so far. I took everyone's suggestions of where to add
> calls to wait_on_page_writeback, which seems to handle the multiple-write case
> adequately. Unfortunately, it is still possible to generate checksum errors by
> scribbling furiously on a mmap'd region, even after adding the writeback wait
> in the ext4 writepage function. Oddly, I couldn't break btrfs with mmap by
> removing its wait_for_page_writeback call, so I suspect there's a bit more
> going on in btrfs than I've been able to figure out.
>
> The set_memory_ro debugging trick didn't ferret out any write paths that I
> didn't catch... though it did have the effect of causing occasional fsync()
> deadlocks. I suppose I could sprinkle in a few more of those write calls to
> see what happens.
>
> Either way, I'm emailing to ask everyone's advice since I've run out of ideas.
> Or: Did I miss something?
>
> Thanks all for the feedback so far!
>
> --
> fs: Wait for page writeback when rewrite detected
>
> Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
> ---
>
> fs/buffer.c | 4 +++-
> fs/ext4/inode.c | 3 +++
> mm/filemap.c | 15 +++++++++++++--
> 3 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 2219a76..39e934c 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2379,8 +2379,10 @@ block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
> ret = VM_FAULT_OOM;
> else /* -ENOSPC, -EIO, etc */
> ret = VM_FAULT_SIGBUS;
> - } else
> + } else {
> + wait_on_page_writeback(page);
> ret = VM_FAULT_LOCKED;
> + }

I think this needs to wait before the __block_write_begin() call,
not after it. i.e. wait before the page is mapped, not afterwards.

....
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 83a45d3..f201d80 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2217,8 +2217,8 @@ EXPORT_SYMBOL(generic_file_direct_write);
> * Find or create a page at the given pagecache position. Return the locked
> * page. This function is specifically for buffered writes.
> */
> -struct page *grab_cache_page_write_begin(struct address_space *mapping,
> - pgoff_t index, unsigned flags)
> +struct page *__grab_cache_page_write_begin(struct address_space *mapping,
> + pgoff_t index, unsigned flags)
> {
> int status;
> struct page *page;
> @@ -2243,6 +2243,17 @@ repeat:
> }
> return page;
> }
> +struct page *grab_cache_page_write_begin(struct address_space *mapping,
> + pgoff_t index, unsigned flags)
> +{
> + struct page *p;
> +
> + p = __grab_cache_page_write_begin(mapping, index, flags);
> + if (p)
> + wait_on_page_writeback(p);
> +
> + return p;
> +}
> EXPORT_SYMBOL(grab_cache_page_write_begin);

Not much point in add in a wrapper when nothing else calls
__grab_cache_page_write_begin(), which should also be static....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com

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