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

Chengming Zhou
2025-02-27T02:30:24Z

On 2025/2/27 07:47, Nhat Pham wrote:
> 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.).

Ah, agree! Actually I also think we should use the zcomp API in zswap,
if its API meets our requirements.

>
> But yes, a different problem for another day :)