Re: [PATCH mmotm] memcg: unmap KM_USER0 at shmem_map_and_free_swp if do_swap_account

[Daisuke Nishimura <d-nishimura@mtf.biglobe.ne.jp>]

On Tue, 18 Nov 2008 12:48:52 +0000 (GMT)
Hugh Dickins <hugh@veritas.com> wrote:

> On Tue, 18 Nov 2008, Daisuke Nishimura wrote:
> 
> > memswap controller uses KM_USER0 at swap_cgroup_record and lookup_swap_cgroup.
> > 
> > But delete_from_swap_cache, which eventually calls swap_cgroup_record, can be
> > called with KM_USER0 mapped in case of shmem.
> > 
> > So it should be unmapped before calling it.
> 
> Excellent find, but ...
> 
> > 
> > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> > ---
> > After this patch, I think memswap controller of x86_32 will be
> > on near level with that of x86_64.
> > 
> >  mm/shmem.c |   23 +++++++++++++++++++++++
> >  1 files changed, 23 insertions(+), 0 deletions(-)
> 
> ... sorry, no, please don't go around unmapping other people's kmaps
> like this.  If the memswap controller needs its own kmap_atomic()s at
> a level below other users, then it needs to define a new KM_MEMSWAP
> in arch/*/include/asm/kmap_types.h and include/asm-*/kmap_types.h.
> 
> That's a lot of files which you may not wish to update to get working
> right now: I think page_cgroup.c can _probably_ reuse KM_PTE1 as a
> temporary measure, but please verify that's safe first.
> 
Thank you for your comment.

Hmm, shmem_map_and_free_swp might unmap dir anyway and caller
(shmem_trancate_range) handles the case, but I do agree it's
not good manner to unmap other people's kmaps.

I'll consider more.
(I don't have enough time till Friday, though.)


Thanks,
Daisuke Nishimura.

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