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.
