linux-kernel - Re: BUG at mm/filemap.c:1749 (2.6.24, jffs2, unionfs)

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Date:	Fri, 19 Oct 2007 17:16:53 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Erez Zadok <ezk@...sunysb.edu>
Cc:	dwmw2@...radead.org, jffs-dev@...s.com,
	linux-kernel@...r.kernel.org, linux-mtd@...ts.infradead.org
Subject: Re: BUG at mm/filemap.c:1749 (2.6.24, jffs2, unionfs)

On Friday 19 October 2007 17:03, Nick Piggin wrote:
> On Friday 19 October 2007 16:05, Erez Zadok wrote:
> > David,
> >
> > I'm testing unionfs on top of jffs2, using 2.6.24 as of linus's commit
> > 4fa4d23fa20de67df919030c1216295664866ad7.  All of my unionfs tests pass
> > when unionfs is stacked on top of jffs2, other than my truncate test --
> > whic tries to truncate files up/down (through the union, which then is
> > passed through to the lower jffs2 f/s).  The same truncate test passes on
> > all other file systems I've tried unionfs/2.6.24 with, as well as all of
> > the earlier kernels that unionfs runs on (2.6.9--2.6.23).  So I tend to
> > think this bug is more probably due to something else going on in 2.6.24,
> > possibly wrt jffs2/mtd.  (Of course, it's still possible that unionfs
> > isn't doing something right -- any pointers?)
> >
> > The oops trace is included below.  Is this a known issue and if so, any
> > fixes?  If this is the first you hear of this problem, let me know and
> > I'll try to narrow it down further.
>
> It's had quite a lot of recent changes in that area -- the "new aops"
> patches.
>
> They've been getting quite a bit of testing in -mm and no such problems,
> but I doubt anyone was doing much unionfs over jffs2, or even much jffs2
> testing with -mm.
>
> The bug smells like jffs2 is actually passing back a "written" length
> greater than the length we passed into it.
>
> The following might show what's happening.

Hmm, looks like jffs2_write_end is writing more than we actually ask it
to, and returns that back.

        unsigned aligned_start = start & ~3;

and

        if (end == PAGE_CACHE_SIZE) {
                /* When writing out the end of a page, write out the
                   _whole_ page. This helps to reduce the number of
                   nodes in files which have many short writes, like
                   syslog files. */
                start = aligned_start = 0;
        }

These "longer" writes are fine, but they shouldn't get propagated back
to the vm/vfs. Something like the following patch might fix it.


View attachment "jffs2-writtenlen-fix.patch" of type "text/x-diff" (1137 bytes)