Re: [PATCH] zswap: initialize entry->pool on same filled entry

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 22, 2024 at 06:35:43AM -0700, Chris Li wrote:
> On Thu, Mar 21, 2024 at 8:19 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Thu, Mar 21, 2024 at 04:56:05PM -0700, Yosry Ahmed wrote:
> > > On Thu, Mar 21, 2024 at 4:53 PM Chris Li <chrisl@kernel.org> wrote:
> > > >
> > > > Current zswap will leave the entry->pool uninitialized if
> > > > the page is same  filled. The entry->pool pointer can
> > > > contain data written by previous usage.
> > > >
> > > > Initialize entry->pool to zero for the same filled zswap entry.
> > > >
> > > > Signed-off-by: Chris Li <chrisl@kernel.org>
> > > > ---
> > > > Per Yosry's suggestion to split out this clean up
> > > > from the zxwap rb tree to xarray patch.
> > > >
> > > > https://lore.kernel.org/all/ZemDuW25YxjqAjm-@google.com/
> > > > ---
> > > >  mm/zswap.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > >
> > > > diff --git a/mm/zswap.c b/mm/zswap.c
> > > > index b31c977f53e9..f04a75a36236 100644
> > > > --- a/mm/zswap.c
> > > > +++ b/mm/zswap.c
> > > > @@ -1527,6 +1527,7 @@ bool zswap_store(struct folio *folio)
> > > >                         kunmap_local(src);
> > > >                         entry->length = 0;
> > > >                         entry->value = value;
> > > > +                       entry->pool = 0;
> > >
> > > This should be NULL.
> > >
> > > That being said, I am working on a series that should make non-filled
> > > entries not use a zswap_entry at all. So I think this cleanup is
> > > unnecessary, especially that it is documented in the definition of
> > > struct zswap_entry that entry->pool is invalid for same-filled
> > > entries.
> >
> > Yeah I don't think it's necessary to initialize. The field isn't valid
> > when it's a same-filled entry, just like `handle` would contain
> > nonsense as it's unionized with value.
> >
> > What would actually be safer is to make the two subtypes explicit, and
> > not have unused/ambiguous/overloaded members at all:
> >
> > struct zswap_entry {
> >         unsigned int length;
> >         struct obj_cgroup *objcg;
> > };
> >
> > struct zswap_compressed_entry {
> >         struct zswap_entry entry;
> >         struct zswap_pool *pool;
> >         unsigned long handle;
> >         struct list_head lru;
> >         swp_entry_t swpentry;
> > };
> >
> > struct zswap_samefilled_entry {
> >         struct zswap_entry entry;
> >         unsigned long value;
> > };
> 
> I think the 3 struct with embedded and container of is a bit complex,
> because the state breaks into different struct members

That's kind of the point. They're different types that have their own
rules and code paths. The code as it is right now makes it seem like
they're almost the same. From the above you can see that they have
actually almost nothing in common (the bits in struct zswap_entry).

This would force the code to show the difference as well.

Depending on how Yosry's patches work out, this may or may not be
worth doing. It's just an idea that could help make it easier.

> How about:
> 
> struct zswap_entry {
>         unsigned int length;
>         struct obj_cgroup *objcg;
>         union {
>                 struct /* compressed */ {
>                          struct zswap_pool *pool;
>                          unsigned long handle;
>                          swp_entry_t swpentry;
>                          struct list_head lru;
>                 };
>                struct /* same filled */ {
>                        unsigned long value;
>                 };
>         };
> };
> 
> That should have the same effect of the above three structures. Easier
> to visualize the containing structure.

I suppose it makes the struct a bit clearer when you directly look at
it, but I don't see how it would help with code clarity.