From: Brendan Jackman Date: Tue, 11 Feb 2025 13:00:23 +0000 (+0000) Subject: mm/mmu_gather: update comment on RCU freeing X-Git-Url: https://www.infradead.org/git/?a=commitdiff_plain;h=fc6158bd23c6a9f7c85f86d93f7a8aaf30a2009c;p=users%2Fjedix%2Flinux-maple.git mm/mmu_gather: update comment on RCU freeing Some recent discussion on LMKL [0] brought up some interesting and useful additional context on RCU-freeing for pagetables. Note down some extra info in here, in particular a) be concrete about the reason why an arch might not have an IPI and b) add the interesting paravirt details. [0] https://lore.kernel.org/linux-kernel/20250206044346.3810242-2-riel@surriel.com/ Link: https://lkml.kernel.org/r/20250211-mmugather-comment-v1-1-1ac1e0c765d2@google.com Signed-off-by: Brendan Jackman Cc: "Aneesh Kumar K.V" Cc: Brendan Jackman Cc: Nicholas Piggin Cc: Peter Zijlstra (Intel) Cc: Rik van Riel Cc: Will Deacon Signed-off-by: Andrew Morton --- diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c index 7aa6f18c500b..db7ba4a725d6 100644 --- a/mm/mmu_gather.c +++ b/mm/mmu_gather.c @@ -246,8 +246,16 @@ static void __tlb_remove_table_free(struct mmu_table_batch *batch) * IRQs delays the completion of the TLB flush we can never observe an already * freed page. * - * Architectures that do not have this (PPC) need to delay the freeing by some - * other means, this is that means. + * Not all systems IPI every CPU for this purpose: + * + * - Some architectures have HW support for cross-CPU synchronisation of TLB + * flushes, so there's no IPI at all. + * + * - Paravirt guests can do this TLB flushing in the hypervisor, or coordinate + * with the hypervisor to defer flushing on preempted vCPUs. + * + * Such systems need to delay the freeing by some other means, this is that + * means. * * What we do is batch the freed directory pages (tables) and RCU free them. * We use the sched RCU variant, as that guarantees that IRQ/preempt disabling