Re: 2.6.20-rc4: known regressions with patches (v3)

[Andrew Morton <akpm@osdl.org>]

On Fri, 12 Jan 2007 08:39:16 +1100
David Chinner <dgc@sgi.com> wrote:

> On Thu, Jan 11, 2007 at 06:13:29AM +0100, Adrian Bunk wrote:
> > This email lists some known regressions in 2.6.20-rc4 compared to 2.6.19
> > with patches available.
> > 
> > Subject    : BUG: at mm/truncate.c:60 cancel_dirty_page()  (XFS)
> > References : http://lkml.org/lkml/2007/1/5/308
> > Submitter  : Sami Farin <7atbggg02@sneakemail.com>
> > Handled-By : David Chinner <dgc@sgi.com>
> > Patch      : http://lkml.org/lkml/2007/1/7/201
> > Status     : patch available
> 
> Patch is broken, do not merge. The original had an off-by-one bug in
> it, and the fixed one I have has just shown a worse problem than
> before - partial page truncation (i.e.  filesystem block size less
> than page size) is busted because invalidate_complete_page2_range() can
> only handle complete pages.
> 
> Andrew - looking at unmap_mapping_pages, it says it cannot handle
> partial pages and must get rid of them whereas vmtrucate() handles
> partial pages but changes file size so can't be used. I see that
> vmtruncate handles this by not unmapping the first partial page.
> 
> I can use the vmtruncate mechanism (unmap_mapping_pages, then
> truncate_inode_pages) but that seems racy to me because we are not
> actually truncating the file so a mmap could remap a page between
> the unmap and the truncate and hence we still get the warning.

Yes, truncate relies upon there being nothing outside i_size, and that
i_mutex is held.

> So the question is - is there any generic function that handles
> this case (i.e. don't unmap first partial page, unmap the rest,
> partial truncate of first page, complete truncate of the rest)
> without racing? Or do I need to write a variation of
> invalidate_inode_pages2_range() to do this?

umm, nothing I can immediately think of.  Perhaps you can generalise
vmtruncate_range() a bit?
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