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

[Yosry Ahmed <yosry.ahmed@linux.dev>]

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?

> 
> 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.

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.