Re: [PATCH 2/2] mm: zswap: remove unnecessary tree cleanups in zswap_swapoff()

[Johannes Weiner]

On Mon, Jan 22, 2024 at 12:39:16PM -0800, Yosry Ahmed wrote:
> On Mon, Jan 22, 2024 at 12:19 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > On Sat, Jan 20, 2024 at 02:40:07AM +0000, Yosry Ahmed wrote:
> > > During swapoff, try_to_unuse() makes sure that zswap_invalidate() is
> > > called for all swap entries before zswap_swapoff() is called. This means
> > > that all zswap entries should already be removed from the tree. Simplify
> > > zswap_swapoff() by removing the tree cleanup loop, and leaving an
> > > assertion in its place.
> > >
> > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
> >
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> >
> > That's a great simplification.
> >
> > Removing the tree->lock made me double take, but at this point the
> > swapfile and its cache should be fully dead and I don't see how any of
> > the zswap operations that take tree->lock could race at this point.
> 
> It took me a while staring at the code to realize this loop is pointless.
> 
> However, while I have your attention on the swapoff path, there's a
> slightly irrelevant problem that I think might be there, but I am not
> sure.
> 
> It looks to me like swapoff can race with writeback, and there may be
> a chance of UAF for the zswap tree. For example, if zswap_swapoff()
> races with shrink_memcg_cb(), I feel like we may free the tree as it
> is being used. For example if zswap_swapoff()->kfree(tree) happen
> right before shrink_memcg_cb()->list_lru_isolate(l, item).
> 
> Please tell me that I am being paranoid and that there is some
> protection against zswap writeback racing with swapoff. It feels like
> we are very careful with zswap entries refcounting, but not with the
> zswap tree itself.

Hm, I don't see how.

Writeback operates on entries from the LRU. By the time
zswap_swapoff() is called, try_to_unuse() -> zswap_invalidate() should
will have emptied out the LRU and tree.

Writeback could have gotten a refcount to the entry and dropped the
tree->lock. But then it does __read_swap_cache_async(), and while
holding the page lock checks the tree under lock once more; if that
finds the entry valid, it means try_to_unuse() hasn't started on this
page yet, and would be held up by the page lock/writeback state.

> > > Chengming, Chris, I think this should make the tree split and the xarray
> > > conversion patches simpler (especially the former). If others agree,
> > > both changes can be rebased on top of this.
> >
> > The resulting code is definitely simpler, but this patch is not a
> > completely trivial cleanup, either. If you put it before Chengming's
> > patch and it breaks something, it would be difficult to pull out
> > without affecting the tree split.
> 
> Are you suggesting I rebase this on top of Chengming's patches? I can
> definitely do this, but the patch will be slightly less
> straightforward, and if the tree split patches break something it
> would be difficult to pull out as well. If you feel like this patch is
> more likely to break things, I can rebase.

Yeah I think it's more subtle. I'd only ask somebody to rebase an
already tested patch on a newer one if the latter were an obvious,
low-risk, prep-style patch. Your patch is good, but it doesn't quite
fit into this particular category, so I'd say no jumping the queue ;)