linux-kernel - Re: [PATCH] mm: fix page_mkclean_one (was: 2.6.19 file content corruption on ext3)

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Date:	Sun, 24 Dec 2006 23:21:13 +0200
From:	"Michael S. Tsirkin" <mst@...lanox.co.il>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Andrei Popa <andrei.popa@...eo.ro>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...l.org>,
	Gordon Farquharson <gordonfarquharson@...il.com>,
	Martin Michlmayr <tbm@...ius.com>,
	Hugh Dickins <hugh@...itas.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	openib-general@...nib.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: fix page_mkclean_one (was: 2.6.19 file content corruption on ext3)

> Quoting Linus Torvalds <torvalds@...l.org>:
> Subject: Re: [PATCH] mm: fix page_mkclean_one (was: 2.6.19 file content corruption on ext3)
>
> Peter, tell me I'm crazy, but with the new rules, the following condition 
> is a bug:
> 
>  - shared mapping
>  - writable
>  - not already marked dirty in the PTE
> 
> because that combination means that the hardware can mark the PTE dirty 
> without us even realizing (and thus not marking the "struct page *" 
> dirty).

Er.
Sorry about bumping in, and I'm not sure I understand all of the discussion,
but this reminded me of an old issue with COW that created what looks
like a vaguely similiar data corruption on infiniband. We solved this for
infiniband with MADV_DONTFORK, but I always wondered why does it not affect
other parts of kernel.  Small reminder from that discussion:

down mmap sem
get user pages
up mmap sem
page becomes shared, and COW (e.g. fork)
process writes to first byte of page <----- gets a copy
Now we had a problem: struct page that we got from get user pages
does not point to a correct page in our process.
For example: if at some point we map this page for DMA, and
hardware writes to last byte of page -----> process does not
see this data.

So for infiniband, what we do is a combination of
- prevent page from becoming COW while hardware might DMA to this page, and
- ask users not to write to page if hardware might DMA to same page
  (even if its using different bytes).

I just wandered - is there some chance something like this could be happening in
the fs code?

HTH,

-- 
MST
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