On Wed, Jan 8, 2025 at 12:34 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > On Tue, Jan 7, 2025 at 9:00 PM Chengming Zhou <chengming.zhou@xxxxxxxxx> wrote: > Please correct me if I am wrong, but my understanding is that memory > allocated with alloc_percpu() is allocated for each *possible* CPU, > and does not go away when CPUs are offlined. We allocate the per-CPU > crypto_acomp_ctx structs with alloc_percpu() (including the mutex), so > they should not go away with CPU offlining. > > OTOH, we allocate the crypto_acomp_ctx.acompx, crypto_acomp_ctx.req, > and crypto_acomp_ctx.buffer only for online CPUs through the CPU > hotplug notifiers (i.e. zswap_cpu_comp_prepare() and > zswap_cpu_comp_dead()). These are the resources that can go away with > CPU offlining, and what we need to protect about. > > The approach I am taking here is to hold the per-CPU mutex in the CPU > offlining code while we free these resources, and set > crypto_acomp_ctx.req to NULL. In acomp_ctx_get_cpu_locked(), we hold > the mutex of the current CPU, and check if crypto_acomp_ctx.req is > NULL. > > If it is NULL, then the CPU is offlined between raw_cpu_ptr() and > acquiring the mutex, and we retry on the new CPU that we end up on. If > it is not NULL, then we are guaranteed that the resources will not be > freed by CPU offlining until acomp_ctx_put_unlock() is called and the > mutex is unlocked. > Ah you're right, that makes a lot of sense now :)
