Re: [RFC] block integrity: Fix write after checksum calculationproblem

From: Jan Kara
Date: Mon Mar 21 2011 - 10:05:03 EST

On Fri 18-03-11 17:07:55, Darrick J. Wong wrote:
> > > 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.
> I wonder, is it possible for this to happen:
> 1. Thread A mmaps a page and tries to write to it. ext4_page_mkwrite executes,
> but there's no ongoing writeback, so it returns without delay.
> 2. Thread A starts writing furiously to the page.
> 3. Thread B runs fsync() or something that results in the page being
> checksummed and scheduled for writeout.
> 4. Thread A continues to write furiously(!) on that same page before the
> controller finishes the DMA transfer.
> 5. Disk gets the page, which now doesn't match its checksum, and *boom*
What happens on writepage (see mm/page-writeback.c:write_cache_pages())
clear_page_dirty_for_io() - removes PageDirty, marks page as read-only in
set_page_writeback() (happens e.g. in __block_write_full_page() called
from filesystem's writepage implementation).

So if you compute the checksum after set_page_writeback() is done in the
writepage() implementation (you cannot use __block_write_full_page() in
that case) and you call wait_on_page_writeback() in ext4_page_mkwrite()
under page lock, you should be safe. If you do all this and still see
errors, something is broken I'd say...

Jan Kara <jack@xxxxxxx>
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