Re: 2.6.19 file content corruption on ext3

[Andrew Morton]

On Mon, 18 Dec 2006 18:22:42 +1100
Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:

> Andrew Morton wrote:
> > On Mon, 18 Dec 2006 15:51:52 +1100
> > Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> > 
> > 
> >>I think the problem Andrew identified is real.
> > 
> > 
> > I don't.  In fact I don't think I described any problem (well, I tried to,
> > but then I contradicted myself).
> 
> By saying that there shouldn't be any dirty ptes if there are no
> dirty buffers? But in that case the _page_ shouldn't be dirty either,
> so that clear_page_dirty would be redundant. But presumably it isn't.

I don't follow that.

The linkage between pte-dirtiness and buffer_heads is a bit hard to follow
without also considering page-dirtiness.

> > Six hours here of fsx-linux plus high memory pressure on SMP on 1k
> > blocksize ext3, mainline.  Zero failures.  It's unlikely that this testing
> > would pass, yet people running normal workloads are able to easily trigger
> > failures.  I suspect we're looking in the wrong place.
> 
> Yes I could believe it the corruption is caused by something else
> completely.

Think so.  We do have a problem here, but only on threaded apps, I believe.
rtorrent doesn't appear to be threaded, and the bug is hit on non-preempt
UP.

> >>The issue is the disconnect between the pte dirtiness and a filesystem
> >>bringing buffers clean.
> > 
> > 
> > Really?  The dirtying direction goes pte_dirty->PG_dirty->BH_Dirty and the
> > cleaning direction goes !BH_Dirty->!PG_dirty->!pte_dirty.  That's pretty
> > simple, setting aside races.
> > 
> > In the try_to_free_buffers case there's a large time inverval between
> > !BH_Dirty and !PG_dirty, but that shouldn't affect anything.
> 
> After try_to_free_buffers detaches the buffers from the page, a
> pagefault can come in, and mark the pte writeable, then set_page_dirty
> (which finds no buffers, so only sets PG_dirty).
> 
> The page can now get dirtied through this mapping.
> 
> try_to_free_buffers then goes on to clean the page and ptes.

try_to_free_buffers() isn't called against a page which doesn't have
buffers.  It'll oops.

> Were you testing with preempt?

nope, just SMP.

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