[PATCH v2] x86/mm/tlb: ignore f->new_tlb_gen when zero

Nadav Amit
2022-07-11T07:03:09Z

From: Nadav Amit <namit@vmware.com>

Commit aa44284960d5 ("x86/mm/tlb: Avoid reading mm_tlb_gen when
possible") introduced an optimization of skipping the flush if the TLB
generation that is flushed (as provided in flush_tlb_info) was already
flushed.

However, arch_tlbbatch_flush() does not provide any generation in
flush_tlb_info. As a result, try_to_unmap_one() would not perform any
TLB flushes.

Fix it by checking whether f->new_tlb_gen is nonzero. Zero value is
anyhow is an invalid generation value. To avoid future confusions,
introduce TLB_GENERATION_INVALID constant and use it properly. Add some
assertions to check no partial flushes are done with
TLB_GENERATION_INVALID or when f->mm is NULL, since this does not make
any sense.

In addition, add the missing unlikely().

Fixes: aa44284960d5 ("x86/mm/tlb: Avoid reading mm_tlb_gen when possible")
Reported-by: Hugh Dickins <hughd@google.com>
Tested-by: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Nadav Amit <namit@vmware.com>

---

v1 -> v2:
* Introduce TLB_GENERATION_INVALID to clarify intent.
* Leave the early return and do not "goto out".
* Add some assertions to check and document in code the relationship
between TLB_GENERATION_INVALID and TLB_FLUSH_ALL.
---
arch/x86/include/asm/tlbflush.h | 1 +
arch/x86/mm/tlb.c | 15 ++++++++++++---
2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
index 4af5579c7ef7..cda3118f3b27 100644
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -16,6 +16,7 @@
void __flush_tlb_all(void);

#define TLB_FLUSH_ALL -1UL
+#define TLB_GENERATION_INVALID 0

void cr4_update_irqsoff(unsigned long set, unsigned long clear);
unsigned long cr4_read_shadow(void);
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index d9314cc8b81f..0f346c51dd99 100644
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -771,7 +771,8 @@ static void flush_tlb_func(void *info)
return;
}

- if (f->new_tlb_gen <= local_tlb_gen) {
+ if (unlikely(f->new_tlb_gen != TLB_GENERATION_INVALID &&
+ f->new_tlb_gen <= local_tlb_gen)) {
/*
* The TLB is already up to date in respect to f->new_tlb_gen.
* While the core might be still behind mm_tlb_gen, checking
@@ -843,6 +844,12 @@ static void flush_tlb_func(void *info)
/* Partial flush */
unsigned long addr = f->start;

+ /* Partial flush cannot have invalid generations */
+ VM_BUG_ON(f->new_tlb_gen == TLB_GENERATION_INVALID);
+
+ /* Partial flush must have valid mm */
+ VM_BUG_ON(f->mm == NULL);
+
nr_invalidate = (f->end - f->start) >> f->stride_shift;

while (addr < f->end) {
@@ -1045,7 +1052,8 @@ void flush_tlb_kernel_range(unsigned long start, unsigned long end)
struct flush_tlb_info *info;

preempt_disable();
- info = get_flush_tlb_info(NULL, start, end, 0, false, 0);
+ info = get_flush_tlb_info(NULL, start, end, 0, false,
+ TLB_GENERATION_INVALID);

on_each_cpu(do_kernel_range_flush, info, 1);

@@ -1214,7 +1222,8 @@ void arch_tlbbatch_flush(struct arch_tlbflush_unmap_batch *batch)

int cpu = get_cpu();

- info = get_flush_tlb_info(NULL, 0, TLB_FLUSH_ALL, 0, false, 0);
+ info = get_flush_tlb_info(NULL, 0, TLB_FLUSH_ALL, 0, false,
+ TLB_GENERATION_INVALID);
/*
* flush_tlb_multi() is not optimized for the common case in which only
* a local TLB flush is needed. Optimize this use-case by calling
--
2.25.1