linux-kernel - Re: [PATCH v4] mm/zswap: invalidate old entry when store fail or !zswap

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Date: Wed, 7 Feb 2024 15:43:08 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: chengming.zhou@...ux.dev
Cc: hannes@...xchg.org, yosryahmed@...gle.com, nphamcs@...il.com,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, Chengming Zhou
 <zhouchengming@...edance.com>, stable@...r.kernel.org, Chris Li
 <chrisl@...nel.org>
Subject: Re: [PATCH v4] mm/zswap: invalidate old entry when store fail or
 !zswap_enabled

On Wed,  7 Feb 2024 11:54:06 +0000 chengming.zhou@...ux.dev wrote:

> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> We may encounter duplicate entry in the zswap_store():
> 
> 1. swap slot that freed to per-cpu swap cache, doesn't invalidate
>    the zswap entry, then got reused. This has been fixed.
> 
> 2. !exclusive load mode, swapin folio will leave its zswap entry
>    on the tree, then swapout again. This has been removed.
> 
> 3. one folio can be dirtied again after zswap_store(), so need to
>    zswap_store() again. This should be handled correctly.
> 
> So we must invalidate the old duplicate entry before insert the
> new one, which actually doesn't have to be done at the beginning
> of zswap_store(). And this is a normal situation, we shouldn't
> WARN_ON(1) in this case, so delete it. (The WARN_ON(1) seems want
> to detect swap entry UAF problem? But not very necessary here.)
> 
> The good point is that we don't need to lock tree twice in the
> store success path.
> 
> Note we still need to invalidate the old duplicate entry in the
> store failure path, otherwise the new data in swapfile could be
> overwrite by the old data in zswap pool when lru writeback.
> 
> We have to do this even when !zswap_enabled since zswap can be
> disabled anytime. If the folio store success before, then got
> dirtied again but zswap disabled, we won't invalidate the old
> duplicate entry in the zswap_store(). So later lru writeback
> may overwrite the new data in swapfile.
> 
> Fixes: 42c06a0e8ebe ("mm: kill frontswap")
> Cc: <stable@...r.kernel.org>

We have a patch ordering issue.

As a cc:stable hotfix, this should be merged into 6.8-rcX and later
backported into -stable trees.  So it will go
mm-hotfixes-unstable->mm-hotfixes-stable->mainline.  So someone has to
make this patch merge and work against latest mm-hotfixes-unstable.

The patch you sent appears to be based on linux-next, so it has
dependencies upon mm-unstable patches which won't be merged into
mainline until the next merge window.

So can you please redo and retest this against mm.git's
mm-hotfixes-unstable branch?  Then I'll try to figure out how to merge
the gigentic pile of mm-unstable zswap changes on top of that.

Thanks.