Re: [PATCH v3 10/12] mempolicy: alloc_pages_mpol() for NUMA policy without vma

[Johannes Weiner]

On Mon, Oct 23, 2023 at 08:10:32PM +0200, domenico cerasuolo wrote:
> Il giorno lun 23 ott 2023 alle ore 19:53 Andrew Morton
> <akpm@xxxxxxxxxxxxxxxxxxxx> ha scritto:
> >
> > On Mon, 23 Oct 2023 18:53:26 +0200 domenico cerasuolo <mimmocerasuolo@xxxxxxxxx> wrote:
> >
> > > > Rebased to mm.git's current mm-stable, to resolve with removal of
> > > > vma_policy() from include/linux/mempolicy.h, and temporary omission
> > > > of Nhat's ZSWAP mods from mm/swap_state.c: no other changes.
> > >
> > > Hi Hugh,
> > >
> > > not sure if it's the rebase, but I don't see an update to
> > > __read_swap_cache_async invocation in zswap.c at line 1078. Shouldn't we pass a
> > > mempolicy there too?
> >
> > No change needed.  zswap_writeback_entry() was passing a NULL for arg
> > `vma' and it's now passing a NULL for arg `mpol'.
> 
> Problem is that alloc_pages_mpol is dereferencing mpol, when I test the zswap
> writeback at 397148729f21edcf700ecb2a01749dbce955d09e it crashes, not sure if
> I'm missing something.

I don't think you are. The NULL vma used to go to get_vma_policy(),
which fell back to

	pol = get_task_policy(current);

Now the NULL pol gets passed to alloc_pages_mpol() directly, which
dereferences it. Oops.

I think Hugh's patch needs zswap to pass get_task_policy(current)
instead of NULL.