The patch titled
Subject: mm/mmu_gather: update comment on RCU freeing
has been added to the -mm mm-unstable branch. Its filename is
mm-mmu_gather-update-comment-on-rcu-freeing.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-mmu_gather-update-comment-on-rcu-freeing.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Brendan Jackman <jackmanb@xxxxxxxxxx>
Subject: mm/mmu_gather: update comment on RCU freeing
Date: Tue, 11 Feb 2025 13:00:23 +0000
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@xxxxxxxxxxx/
Link: https://lkml.kernel.org/r/20250211-mmugather-comment-v1-1-1ac1e0c765d2@xxxxxxxxxx
Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxx>
Cc: Brendan Jackman <jackmanb@xxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/mmu_gather.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
--- a/mm/mmu_gather.c~mm-mmu_gather-update-comment-on-rcu-freeing
+++ a/mm/mmu_gather.c
@@ -246,8 +246,16 @@ static void __tlb_remove_table_free(stru
* 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
_
Patches currently in -mm which might be from jackmanb@xxxxxxxxxx are
mm-mmu_gather-update-comment-on-rcu-freeing.patch
