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From: Yosry Ahmed <yosryahmed@google.com>
Date: Fri, 22 Mar 2024 11:58:12 -0700
Message-ID: <CAJD7tkZk=Febdoa-fXVw2n21KtEH=y8n6LD6akxc_cHvfEZ=3g@mail.gmail.com>
Subject: Re: [PATCH] zswap: initialize entry->pool on same filled entry
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Chris Li <chrisl@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, 
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, 
	Nhat Pham <nphamcs@gmail.com>, Chengming Zhou <zhouchengming@bytedance.com>
Content-Type: text/plain; charset="UTF-8"

[..]
> > > 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.

I initially wanted to do something similar to splitting the structs
before not allocating an entry at all for same-filled pages, but I
ended up dropping it as the direct conversion was simple enough.

Anyway, I will post the patches some time next week (or today if I can
get around to test them). The discussion should be easier with code.