Re: [PATCH v2] mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()

[Nhat Pham <nphamcs@gmail.com>]

On Wed, Feb 26, 2025 at 1:23 PM Yosry Ahmed <yosry.ahmed@linux.dev> wrote:
>
> On Wed, Feb 26, 2025 at 09:16:28PM +0000, Eric Biggers wrote:
> > On Wed, Feb 26, 2025 at 08:32:22PM +0000, Yosry Ahmed wrote:
> > > On Wed, Feb 26, 2025 at 08:00:16PM +0000, Eric Biggers wrote:
> > > > On Wed, Feb 26, 2025 at 06:56:25PM +0000, Yosry Ahmed wrote:
> > > > > Currently, zswap_cpu_comp_dead() calls crypto_free_acomp() while holding
> > > > > the per-CPU acomp_ctx mutex. crypto_free_acomp() then holds scomp_lock
> > > > > (through crypto_exit_scomp_ops_async()).
> > > > >
> > > > > On the other hand, crypto_alloc_acomp_node() holds the scomp_lock
> > > > > (through crypto_scomp_init_tfm()), and then allocates memory.
> > > > > If the allocation results in reclaim, we may attempt to hold the per-CPU
> > > > > acomp_ctx mutex.
> > > >
> > > > The bug is in acomp.  crypto_free_acomp() should never have to wait for a memory
> > > > allocation.  That is what needs to be fixed.
> > >
> > > crypto_free_acomp() does not explicitly wait for an allocation, but it
> > > waits for scomp_lock (in crypto_exit_scomp_ops_async()), which may be
> > > held while allocating memory from crypto_scomp_init_tfm().
> > >
> > > Are you suggesting that crypto_exit_scomp_ops_async() should not be
> > > holding scomp_lock?
> >
> > I think the solution while keeping the bounce buffer in place would be to do
> > what the patch
> > https://lore.kernel.org/linux-crypto/Z6w7Pz8jBeqhijut@gondor.apana.org.au/ does,
> > i.e. make the actual allocation and free happen outside the lock.
>
> I am fine with a solution like that if Herbert is fine with it. Although
> as I mentioned, I think this patch is nice to have anyway.
>
> >
> > > > But really the bounce buffering in acomp (which is what is causing this problem)
> > > > should not exist at all.  There is really no practical use case for it; it's
> > > > just there because of the Crypto API's insistence on shoehorning everything into
> > > > scatterlists for no reason...
> > >
> > > I am assuming this about scomp_scratch logic, which is what we need to
> > > hold the scomp_lock for, resulting in this problem.
> >
> > Yes.
> >
> > > If this is something that can be done right away I am fine with dropping
> > > this patch for an alternative fix, although it may be nice to reduce the
> > > lock critical section in zswap_cpu_comp_dead() to the bare minimum
> > > anyway.
> >
> > Well, unfortunately the whole Crypto API philosophy of having a single interface
> > for software and for hardware offload doesn't really work.  This is just yet
> > another example of that; it's a problem caused by shoehorning software
> > compression into an interface designed for hardware offload.  zcomp really
> > should just use the compression libs directly (like most users of compression in
> > the kernel already do), and have an alternate code path specifically for
> > hardware offload (using acomp) for the few people who really want that.
>
> zcomp is for zram, zswap does not use it. If zswap is not going to use
> the crypto API we'll want something like zcomp or maybe reuse zcomp
> itself. That's a problem for another day :)

I'm actually thinking whether we should expose the zcomp API and use
it for zswap. There are a couple of parameters for zstd I wanna play
with, which zcomp/zram seems to already support, but not the crypto
API (zstd level, dictionary, etc.).

But yes, a different problem for another day :)