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SubjectRe: msync(2) bug(?), returns AOP_WRITEPAGE_ACTIVATE to userland
In message <Pine.LNX.4.64.0710142049000.13119@sbz-30.cs.Helsinki.FI>, Pekka J Enberg writes:
> Hi Erez,
>
> On Sun, 14 Oct 2007, Erez Zadok wrote:
> > In unionfs_writepage() I tried to emulate as best possible what the lower
> > f/s will have returned to the VFS. Since tmpfs's ->writepage can return
> > AOP_WRITEPAGE_ACTIVATE and re-mark its page as dirty, I did the same in
> > unionfs: mark again my page as dirty, and return AOP_WRITEPAGE_ACTIVATE.
> >
> > Should I be doing something different when unionfs stacks on top of tmpfs?
> > (BTW, this is probably also relevant to ecryptfs.)
>
> 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?
>
> Pekka
[...]

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.

Erez.
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