Re: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland

[Hugh Dickins <hugh@veritas.com>]

On Sun, 14 Oct 2007, Erez Zadok wrote:
> In message <Pine.LNX.4.64.0710142049000.13119@sbz-30.cs.Helsinki.FI>, Pekka J Enberg writes:
> > 
> > Look at mm/filemap.c:__filemap_fdatawrite_range(). You shouldn't be 
> > calling unionfs_writepage() _at all_ if the lower mapping has 
> > BDI_CAP_NO_WRITEBACK capability set. Perhaps something like the totally 
> > untested patch below?
...

I don't disagree with your unionfs_writepages patch, Pekka, but I think
it should be viewed as an optimization (don't waste time trying to write
a group of pages when we know that nothing will be done) rather than as
essential.

Prior to unionfs's own use of AOP_WRITEPAGE_ACTIVATE, there have only
been ramdisk and shmem generating it.  ramdisk is careful only to
return it in the wbc->for_reclaim case: I think (as in the patch
I sent out before) shmem now ought to do so too for safety.

Back in 2.4 days it was reasonable to assume that ->writepage would
only get called from certain places, but things move faster nowadays,
and the unionfs example shows others are liable to start ab/using it.
I'll send Andrew that patch tomorrow (it's simple enough, but I'd
like at least to try to reproduce the page_mapped bug first).

> 
> Pekka, with a small change to your patch (to handle time-based cache
> coherency), your patch worked well and passed all my tests.  Thanks.
> 
> So now I wonder if we still need the patch to prevent AOP_WRITEPAGE_ACTIVATE
> from being returned to userland.  I guess we still need it, b/c even with
> your patch, generic_writepages() can return AOP_WRITEPAGE_ACTIVATE back to
> the VFS and we need to ensure that doesn't "leak" outside the kernel.

Can it now?  Current git has a patch from Andrew which bears a striking
resemblance to that from Pekka, stopping the leak from write_cache_pages.

Hugh