[PATCH 1/4] x86/mm/tlb: Revert the recent lazy TLB patches

From: Peter Zijlstra
Date: Wed Aug 22 2018 - 11:46:11 EST

Revert commits:

95b0e6357d3e x86/mm/tlb: Always use lazy TLB mode
64482aafe55f x86/mm/tlb: Only send page table free TLB flush to lazy TLB CPUs
ac0315896970 x86/mm/tlb: Make lazy TLB mode lazier
61d0beb5796a x86/mm/tlb: Restructure switch_mm_irqs_off()
2ff6ddf19c0e x86/mm/tlb: Leave lazy TLB mode at page table free time

In order to simplify the TLB invalidate fixes for x86. We'll try again later.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
--- a/arch/x86/include/asm/tlbflush.h
+++ b/arch/x86/include/asm/tlbflush.h
@@ -148,6 +148,22 @@ static inline unsigned long build_cr3_no
#define __flush_tlb_one_user(addr) __native_flush_tlb_one_user(addr)
#endif

+static inline bool tlb_defer_switch_to_init_mm(void)
+{
+ /*
+ * If we have PCID, then switching to init_mm is reasonably
+ * fast. If we don't have PCID, then switching to init_mm is
+ * quite slow, so we try to defer it in the hopes that we can
+ * avoid it entirely. The latter approach runs the risk of
+ * receiving otherwise unnecessary IPIs.
+ *
+ * This choice is just a heuristic. The tlb code can handle this
+ * function returning true or false regardless of whether we have
+ * PCID.
+ */
+ return !static_cpu_has(X86_FEATURE_PCID);
+}
+
struct tlb_context {
u64 ctx_id;
u64 tlb_gen;
@@ -538,9 +554,4 @@ extern void arch_tlbbatch_flush(struct a
native_flush_tlb_others(mask, info)
#endif

-extern void tlb_flush_remove_tables(struct mm_struct *mm);
-extern void tlb_flush_remove_tables_local(void *arg);
-
-#define HAVE_TLB_FLUSH_REMOVE_TABLES
-
#endif /* _ASM_X86_TLBFLUSH_H */
--- a/arch/x86/mm/tlb.c
+++ b/arch/x86/mm/tlb.c
@@ -7,7 +7,6 @@
#include <linux/export.h>
#include <linux/cpu.h>
#include <linux/debugfs.h>
-#include <linux/gfp.h>

#include <asm/tlbflush.h>
#include <asm/mmu_context.h>
@@ -186,11 +185,8 @@ void switch_mm_irqs_off(struct mm_struct
{
struct mm_struct *real_prev = this_cpu_read(cpu_tlbstate.loaded_mm);
u16 prev_asid = this_cpu_read(cpu_tlbstate.loaded_mm_asid);
- bool was_lazy = this_cpu_read(cpu_tlbstate.is_lazy);
unsigned cpu = smp_processor_id();
u64 next_tlb_gen;
- bool need_flush;
- u16 new_asid;

/*
* NB: The scheduler will call us with prev == next when switching
@@ -244,41 +240,20 @@ void switch_mm_irqs_off(struct mm_struct
next->context.ctx_id);

/*
- * Even in lazy TLB mode, the CPU should stay set in the
- * mm_cpumask. The TLB shootdown code can figure out from
- * from cpu_tlbstate.is_lazy whether or not to send an IPI.
+ * We don't currently support having a real mm loaded without
+ * our cpu set in mm_cpumask(). We have all the bookkeeping
+ * in place to figure out whether we would need to flush
+ * if our cpu were cleared in mm_cpumask(), but we don't
+ * currently use it.
*/
if (WARN_ON_ONCE(real_prev != &init_mm &&
!cpumask_test_cpu(cpu, mm_cpumask(next))))
cpumask_set_cpu(cpu, mm_cpumask(next));

- /*
- * If the CPU is not in lazy TLB mode, we are just switching
- * from one thread in a process to another thread in the same
- * process. No TLB flush required.
- */
- if (!was_lazy)
- return;
-
- /*
- * Read the tlb_gen to check whether a flush is needed.
- * If the TLB is up to date, just use it.
- * The barrier synchronizes with the tlb_gen increment in
- * the TLB shootdown code.
- */
- smp_mb();
- next_tlb_gen = atomic64_read(&next->context.tlb_gen);
- if (this_cpu_read(cpu_tlbstate.ctxs[prev_asid].tlb_gen) ==
- next_tlb_gen)
- return;
-
- /*
- * TLB contents went out of date while we were in lazy
- * mode. Fall through to the TLB switching code below.
- */
- new_asid = prev_asid;
- need_flush = true;
+ return;
} else {
+ u16 new_asid;
+ bool need_flush;
u64 last_ctx_id = this_cpu_read(cpu_tlbstate.last_ctx_id);

/*
@@ -329,41 +304,41 @@ void switch_mm_irqs_off(struct mm_struct
next_tlb_gen = atomic64_read(&next->context.tlb_gen);

choose_new_asid(next, next_tlb_gen, &new_asid, &need_flush);
- }

- if (need_flush) {
- this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id);
- this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen);
- load_new_mm_cr3(next->pgd, new_asid, true);
+ if (need_flush) {
+ this_cpu_write(cpu_tlbstate.ctxs[new_asid].ctx_id, next->context.ctx_id);
+ this_cpu_write(cpu_tlbstate.ctxs[new_asid].tlb_gen, next_tlb_gen);
+ load_new_mm_cr3(next->pgd, new_asid, true);
+
+ /*
+ * NB: This gets called via leave_mm() in the idle path
+ * where RCU functions differently. Tracing normally
+ * uses RCU, so we need to use the _rcuidle variant.
+ *
+ * (There is no good reason for this. The idle code should
+ * be rearranged to call this before rcu_idle_enter().)
+ */
+ trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
+ } else {
+ /* The new ASID is already up to date. */
+ load_new_mm_cr3(next->pgd, new_asid, false);
+
+ /* See above wrt _rcuidle. */
+ trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, 0);
+ }

/*
- * NB: This gets called via leave_mm() in the idle path
- * where RCU functions differently. Tracing normally
- * uses RCU, so we need to use the _rcuidle variant.
- *
- * (There is no good reason for this. The idle code should
- * be rearranged to call this before rcu_idle_enter().)
+ * Record last user mm's context id, so we can avoid
+ * flushing branch buffer with IBPB if we switch back
+ * to the same user.
*/
- trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
- } else {
- /* The new ASID is already up to date. */
- load_new_mm_cr3(next->pgd, new_asid, false);
+ if (next != &init_mm)
+ this_cpu_write(cpu_tlbstate.last_ctx_id, next->context.ctx_id);

- /* See above wrt _rcuidle. */
- trace_tlb_flush_rcuidle(TLB_FLUSH_ON_TASK_SWITCH, 0);
+ this_cpu_write(cpu_tlbstate.loaded_mm, next);
+ this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
}

- /*
- * Record last user mm's context id, so we can avoid
- * flushing branch buffer with IBPB if we switch back
- * to the same user.
- */
- if (next != &init_mm)
- this_cpu_write(cpu_tlbstate.last_ctx_id, next->context.ctx_id);
-
- this_cpu_write(cpu_tlbstate.loaded_mm, next);
- this_cpu_write(cpu_tlbstate.loaded_mm_asid, new_asid);
-
load_mm_cr4(next);
switch_ldt(real_prev, next);
}
@@ -386,7 +361,20 @@ void enter_lazy_tlb(struct mm_struct *mm
if (this_cpu_read(cpu_tlbstate.loaded_mm) == &init_mm)
return;

- this_cpu_write(cpu_tlbstate.is_lazy, true);
+ if (tlb_defer_switch_to_init_mm()) {
+ /*
+ * There's a significant optimization that may be possible
+ * here. We have accurate enough TLB flush tracking that we
+ * don't need to maintain coherence of TLB per se when we're
+ * lazy. We do, however, need to maintain coherence of
+ * paging-structure caches. We could, in principle, leave our
+ * old mm loaded and only switch to init_mm when
+ * tlb_remove_page() happens.
+ */
+ this_cpu_write(cpu_tlbstate.is_lazy, true);
+ } else {
+ switch_mm(NULL, &init_mm, NULL);
+ }
}

/*
@@ -473,9 +461,6 @@ static void flush_tlb_func_common(const
* paging-structure cache to avoid speculatively reading
* garbage into our TLB. Since switching to init_mm is barely
* slower than a minimal flush, just switch to init_mm.
- *
- * This should be rare, with native_flush_tlb_others skipping
- * IPIs to lazy TLB mode CPUs.
*/
switch_mm_irqs_off(NULL, &init_mm, NULL);
return;
@@ -582,9 +567,6 @@ static void flush_tlb_func_remote(void *
void native_flush_tlb_others(const struct cpumask *cpumask,
const struct flush_tlb_info *info)
{
- cpumask_var_t lazymask;
- unsigned int cpu;
-
count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
if (info->end == TLB_FLUSH_ALL)
trace_tlb_flush(TLB_REMOTE_SEND_IPI, TLB_FLUSH_ALL);
@@ -608,6 +590,8 @@ void native_flush_tlb_others(const struc
* that UV should be updated so that smp_call_function_many(),
* etc, are optimal on UV.
*/
+ unsigned int cpu;
+
cpu = smp_processor_id();
cpumask = uv_flush_tlb_others(cpumask, info);
if (cpumask)
@@ -615,29 +599,8 @@ void native_flush_tlb_others(const struc
(void *)info, 1);
return;
}
-
- /*
- * A temporary cpumask is used in order to skip sending IPIs
- * to CPUs in lazy TLB state, while keeping them in mm_cpumask(mm).
- * If the allocation fails, simply IPI every CPU in mm_cpumask.
- */
- if (!alloc_cpumask_var(&lazymask, GFP_ATOMIC)) {
- smp_call_function_many(cpumask, flush_tlb_func_remote,
+ smp_call_function_many(cpumask, flush_tlb_func_remote,
(void *)info, 1);
- return;
- }
-
- cpumask_copy(lazymask, cpumask);
-
- for_each_cpu(cpu, lazymask) {
- if (per_cpu(cpu_tlbstate.is_lazy, cpu))
- cpumask_clear_cpu(cpu, lazymask);
- }
-
- smp_call_function_many(lazymask, flush_tlb_func_remote,
- (void *)info, 1);
-
- free_cpumask_var(lazymask);
}

/*
@@ -690,68 +653,6 @@ void flush_tlb_mm_range(struct mm_struct
put_cpu();
}

-void tlb_flush_remove_tables_local(void *arg)
-{
- struct mm_struct *mm = arg;
-
- if (this_cpu_read(cpu_tlbstate.loaded_mm) == mm &&
- this_cpu_read(cpu_tlbstate.is_lazy)) {
- /*
- * We're in lazy mode. We need to at least flush our
- * paging-structure cache to avoid speculatively reading
- * garbage into our TLB. Since switching to init_mm is barely
- * slower than a minimal flush, just switch to init_mm.
- */
- switch_mm_irqs_off(NULL, &init_mm, NULL);
- }
-}
-
-static void mm_fill_lazy_tlb_cpu_mask(struct mm_struct *mm,
- struct cpumask *lazy_cpus)
-{
- int cpu;
-
- for_each_cpu(cpu, mm_cpumask(mm)) {
- if (!per_cpu(cpu_tlbstate.is_lazy, cpu))
- cpumask_set_cpu(cpu, lazy_cpus);
- }
-}
-
-void tlb_flush_remove_tables(struct mm_struct *mm)
-{
- int cpu = get_cpu();
- cpumask_var_t lazy_cpus;
-
- if (cpumask_any_but(mm_cpumask(mm), cpu) >= nr_cpu_ids) {
- put_cpu();
- return;
- }
-
- if (!zalloc_cpumask_var(&lazy_cpus, GFP_ATOMIC)) {
- /*
- * If the cpumask allocation fails, do a brute force flush
- * on all the CPUs that have this mm loaded.
- */
- smp_call_function_many(mm_cpumask(mm),
- tlb_flush_remove_tables_local, (void *)mm, 1);
- put_cpu();
- return;
- }
-
- /*
- * CPUs with !is_lazy either received a TLB flush IPI while the user
- * pages in this address range were unmapped, or have context switched
- * and reloaded %CR3 since then.
- *
- * Shootdown IPIs at page table freeing time only need to be sent to
- * CPUs that may have out of date TLB contents.
- */
- mm_fill_lazy_tlb_cpu_mask(mm, lazy_cpus);
- smp_call_function_many(lazy_cpus,
- tlb_flush_remove_tables_local, (void *)mm, 1);
- free_cpumask_var(lazy_cpus);
- put_cpu();
-}

static void do_flush_tlb_all(void *info)
{
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -303,14 +303,4 @@ static inline void tlb_remove_check_page

#define tlb_migrate_finish(mm) do {} while (0)

-/*
- * Used to flush the TLB when page tables are removed, when lazy
- * TLB mode may cause a CPU to retain intermediate translations
- * pointing to about-to-be-freed page table memory.
- */
-#ifndef HAVE_TLB_FLUSH_REMOVE_TABLES
-#define tlb_flush_remove_tables(mm) do {} while (0)
-#define tlb_flush_remove_tables_local(mm) do {} while (0)
-#endif
-
#endif /* _ASM_GENERIC__TLB_H */
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -326,20 +326,16 @@ bool __tlb_remove_page_size(struct mmu_g

#ifdef CONFIG_HAVE_RCU_TABLE_FREE

+/*
+ * See the comment near struct mmu_table_batch.
+ */
+
static void tlb_remove_table_smp_sync(void *arg)
{
- struct mm_struct __maybe_unused *mm = arg;
- /*
- * On most architectures this does nothing. Simply delivering the
- * interrupt is enough to prevent races with software page table
- * walking like that done in get_user_pages_fast.
- *
- * See the comment near struct mmu_table_batch.
- */
- tlb_flush_remove_tables_local(mm);
+ /* Simply deliver the interrupt */
}

-static void tlb_remove_table_one(void *table, struct mmu_gather *tlb)
+static void tlb_remove_table_one(void *table)
{
/*
* This isn't an RCU grace period and hence the page-tables cannot be
@@ -348,7 +344,7 @@ static void tlb_remove_table_one(void *t
* It is however sufficient for software page-table walkers that rely on
* IRQ disabling. See the comment near struct mmu_table_batch.
*/
- smp_call_function(tlb_remove_table_smp_sync, tlb->mm, 1);
+ smp_call_function(tlb_remove_table_smp_sync, NULL, 1);
__tlb_remove_table(table);
}

@@ -369,8 +365,6 @@ void tlb_table_flush(struct mmu_gather *
{
struct mmu_table_batch **batch = &tlb->batch;

- tlb_flush_remove_tables(tlb->mm);
-
if (*batch) {
call_rcu_sched(&(*batch)->rcu, tlb_remove_table_rcu);
*batch = NULL;
@@ -393,7 +387,7 @@ void tlb_remove_table(struct mmu_gather
if (*batch == NULL) {
*batch = (struct mmu_table_batch *)__get_free_page(GFP_NOWAIT | __GFP_NOWARN);
if (*batch == NULL) {
- tlb_remove_table_one(table, tlb);
+ tlb_remove_table_one(table);
return;
}
(*batch)->nr = 0;