[PATCH v3 1/7] alpha: run check_mmu_context() from finish_arch_post_lock_switch()

[PATCH v3 1/7] alpha: run check_mmu_context() from finish_arch_post_lock_switch()

[Magnus Lindholm]

check_mmu_context() clears asn_lock and acts on need_new_asn, but it
runs only as the tail of switch_to(), after alpha_switch_to() returns.
A newly forked task never gets there: its first context switch resumes
at ret_from_fork, which goes to schedule_tail() and then to user space
rather than returning to the code following alpha_switch_to(). A new
kernel thread reaches schedule_tail() the same way, through
ret_from_kernel_thread().

asn_lock is left set on that CPU, so the forked task runs user space
with it set and interrupts enabled. A TLB shootdown IPI arriving in
that window takes the deferred path, and the need_new_asn handshake
meant to cover that never runs.

finish_task_switch() calls finish_arch_post_lock_switch() with
preemption disabled, on the CPU that ran switch_mm(), so hooking
check_mmu_context() there completes the bookkeeping for both. The
existing call from switch_to() then becomes redundant, since
finish_task_switch() runs immediately afterwards and does the same
work, so drop it.

kthread_use_mm() and sched_force_init_mm() reach the same hook outside
the scheduler's preemption-disabled switch tail, where the CPU may have
changed since switch_mm(). check_mmu_context() acts on per-CPU state,
so testing preemptible() expresses the required condition directly
rather than naming particular callers. alpha selects ARCH_NO_PREEMPT,
so unless something else turns on PREEMPT_COUNT the test is a
compile-time 0 and the hook runs everywhere, including at the end of
kthread_use_mm(); it only takes effect in PREEMPT_COUNT builds.

Moving the call changes when it runs. switch_to() ran it with the rq
lock held and interrupts off, while finish_arch_post_lock_switch() runs
after finish_lock_switch() has dropped that lock and re-enabled
interrupts, with preemption still disabled. A shootdown IPI taken in
that window either finds asn_lock still set and defers, or finds it
already cleared and flushes directly and by then PAL_swpctx has
installed the incoming context, so the direct flush acts on the right
one. need_new_asn is only ever set while asn_lock is 1.

It also changes one case for the worse, which the series that follows
does not fix. Before this patch nothing cleared asn_lock during a
kthread_use_mm() borrow, since check_mmu_context() ran only from
switch_to(), so a shootdown IPI for the borrowed mm found asn_locked()
true and took the conservative flush_tlb_other() path, retiring
mm->context[cpu]. With the hook in place asn_lock is cleared when
kthread_use_mm() returns, so such an IPI instead issues a targeted tbi()
against a context that ev5_switch_mm() prepared but never installed, and
leaves the slot valid. Loading the context on a direct mm switch closes
that window; where both changes are applied, that one belongs first.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/include/asm/mmu_context.h | 8 ++++++++
 arch/alpha/include/asm/switch_to.h   | 1 -
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/include/asm/mmu_context.h b/arch/alpha/include/asm/mmu_context.h
index eee8fe836a59..825d3b9605c9 100644
--- a/arch/alpha/include/asm/mmu_context.h
+++ b/arch/alpha/include/asm/mmu_context.h
@@ -181,6 +181,14 @@ do {								\
 #define check_mmu_context()  do { } while(0)
 #endif
 
+/* Per-CPU state: only safe while still on the switching CPU.  */
+#define finish_arch_post_lock_switch finish_arch_post_lock_switch
+static inline void finish_arch_post_lock_switch(void)
+{
+	if (!preemptible())
+		check_mmu_context();
+}
+
 __EXTERN_INLINE void
 ev5_activate_mm(struct mm_struct *prev_mm, struct mm_struct *next_mm)
 {
diff --git a/arch/alpha/include/asm/switch_to.h b/arch/alpha/include/asm/switch_to.h
index 762b7f975310..35c4b2c9d992 100644
--- a/arch/alpha/include/asm/switch_to.h
+++ b/arch/alpha/include/asm/switch_to.h
@@ -9,7 +9,6 @@ extern struct task_struct *alpha_switch_to(unsigned long, struct task_struct *);
 #define switch_to(P,N,L)						 \
   do {									 \
     (L) = alpha_switch_to(virt_to_phys(&task_thread_info(N)->pcb), (P)); \
-    check_mmu_context();						 \
   } while (0)
 
 #endif /* __ALPHA_SWITCH_TO_H */
-- 
2.43.0

Re: [PATCH v3 1/7] alpha: run check_mmu_context() from finish_arch_post_lock_switch()

[Matt Turner]

On Wed, Sep 23, 2026 at 09:47:44AM +0200, Magnus Lindholm wrote:
> so testing preemptible() expresses the required condition directly
> rather than naming particular callers. alpha selects ARCH_NO_PREEMPT,
> so unless something else turns on PREEMPT_COUNT the test is a
> compile-time 0 and the hook runs everywhere, including at the end of
> kthread_use_mm(); it only takes effect in PREEMPT_COUNT builds.

The only way I can find to get PREEMPT_COUNT on alpha is
RCU_STRICT_GRACE_PERIOD. DEBUG_ATOMIC_SLEEP depends on !ARCH_NO_PREEMPT
and PROVE_LOCKING only selects it if !ARCH_NO_PREEMPT.

In that config preemptible() is true at the end of kthread_use_mm(), but
the task still cannot migrate there: there is no kernel preemption and
nothing sleeps between switch_mm_irqs_off() and the hook. So the test
never prevents anything, and its only effect is that one debug config
leaves asn_lock set where every other config clears it. I would drop it.

> A shootdown IPI taken in
> that window either finds asn_lock still set and defers, or finds it
> already cleared and flushes directly and by then PAL_swpctx has
> installed the incoming context, so the direct flush acts on the right
> one. need_new_asn is only ever set while asn_lock is 1.

The deferred case has two halves. If ev5_switch_mm() reused the ASN,
need_new_asn is set and check_mmu_context() reloads. If it allocated a
new one, need_new_asn is 0, the IPI zeroes mm->context[cpu], and the
task runs on with a zero slot. Nothing is stale at that point, but the
follow-up series reads a zero slot as "not running here". More in my
reply to v2 3/3 there.

[PATCH v3 2/7] alpha: only use a targeted tbi() when the target mm is really current

[Magnus Lindholm]

ipi_flush_tlb_page() gates a targeted tbi() on current->active_mm.
tbi() acts on the address space context currently loaded on the CPU, so
it is only guaranteed to reach an mm's translations when a thread of
that mm is running there. current->active_mm is not sufficient: under
lazy TLB an idle or kernel task keeps an mm as its active_mm while a
different ASN is loaded, so the invalidate is issued against the wrong
context and the stale entry can survive. Nothing retires the old ASN
afterwards either, mm->context[cpu] still being valid, so the resuming
thread can reuse it.

Test current->mm instead and otherwise fall back to flush_tlb_other(),
which clears mm->context[cpu] and forces a fresh ASN at the next switch
whatever is loaded now.

This does not cover a task that borrowed an mm through
kthread_use_mm(): both comparisons match such a task, which has
current->mm set while ev5_switch_mm() has only prepared the PCB and
nothing has issued the PAL_swpctx that installs it. asn_locked() does
not identify that window either, for the reason given in the previous
patch. Loading the context on a direct mm switch is what fixes that
case, and is a separate change.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index ed06367ece57..1ad448105201 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -669,7 +669,8 @@ ipi_flush_tlb_page(void *x)
 	struct flush_tlb_page_struct *data = x;
 	struct mm_struct * mm = data->mm;
 
-	if (mm == current->active_mm && !asn_locked())
+	/* A targeted tbi() needs a thread of MM to be current.  */
+	if (mm == current->mm && !asn_locked())
 		flush_tlb_current_page(mm, data->vma, data->addr);
 	else
 		flush_tlb_other(mm);
-- 
2.43.0

[PATCH v3 3/7] alpha: fix the local TLB invalidate in flush_tlb_page()

[Magnus Lindholm]

flush_tlb_page() invalidates the calling CPU itself before asking the
others, and gates that on current->active_mm. For a non-executable vma
that means a targeted tbi(2, addr), which acts on the context currently
loaded and is only guaranteed to invalidate the intended translations
when the target mm's context is the loaded one. A lazy active_mm does
not establish that, so as in ipi_flush_tlb_page() the target mm's stale
translations can survive, and nothing forces the old ASN to be retired
afterwards.

Test current->mm instead. The calling CPU then has no local invalidate
at all when the mm is only lazily borrowed; the next patch adds it.

Which callers reach here with a foreign mm depends on the configuration.
folio_mkclean(), from the writeback flusher kworker that has no mm of its
own, accounted for about half the calls during writeback of a shared
mapping and none at all on anonymous memory. Those counts were measured
with CONFIG_COMPACTION=n: with COMPACTION=y, asm/pgtable.h overrides
ptep_clear_flush() to call migrate_flush_tlb_page(), which rendezvouses
with every CPU and handles the context itself, so folio_mkclean() does
not reach this function.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 1ad448105201..7856d23b3384 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -684,7 +684,8 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 
 	preempt_disable();
 
-	if (mm == current->active_mm) {
+	/* As in ipi_flush_tlb_page(): a targeted tbi() needs MM current.  */
+	if (mm == current->mm) {
 		flush_tlb_current_page(mm, vma, addr);
 		if (atomic_read(&mm->mm_users) <= 1) {
 			int cpu, this_cpu = smp_processor_id();
-- 
2.43.0

Re: [PATCH v3 3/7] alpha: fix the local TLB invalidate in flush_tlb_page()

[Matt Turner]

On Wed, Sep 23, 2026 at 09:47:46AM +0200, Magnus Lindholm wrote:
> Test current->mm instead. The calling CPU then has no local invalidate
> at all when the mm is only lazily borrowed; the next patch adds it.

Please swap this with 4/7. The else branch is correct on its own, since
it only covers mm != active_mm until this patch widens it. In the
current order a bisect that lands here has a lazy caller doing nothing
locally, which is worse than before on a machine where the tbi()
against the wrong ASN happens to work, as it appears to on EV7.

[PATCH v3 4/7] alpha: invalidate the local context in flush_tlb_page()

[Magnus Lindholm]

When the target mm is not current, flush_tlb_page() does nothing locally,
and smp_call_function() reaches only the other CPUs. This CPU may still
hold translations for the mm and can later reuse the old ASN together with
them.

Add the missing else branch. flush_tlb_other() clears mm->context[cpu] so
a fresh ASN is taken at the next switch. With the previous patch this also
covers a lazily borrowed mm, which no longer takes the targeted path.

The uniprocessor implementations of flush_tlb_mm() and
flush_icache_user_page() in asm/tlbflush.h and asm/cacheflush.h already
contain exactly this branch.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 7856d23b3384..a5a42ae4a7d8 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -698,6 +698,9 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 			preempt_enable();
 			return;
 		}
+	} else {
+		/* smp_call_function() does not call back into this CPU.  */
+		flush_tlb_other(mm);
 	}
 
 	data.vma = vma;
-- 
2.43.0

[PATCH v3 5/7] alpha: fix the local TLB invalidate in the UP flush_tlb_page()

[Magnus Lindholm]

The uniprocessor flush_tlb_page() in asm/tlbflush.h has the same wrong
test as the SMP one: a targeted tbi() is only guaranteed to invalidate
the intended translations when the target mm's context is loaded, but
the gate is current->active_mm, which a lazily borrowed mm also
satisfies.

Test current->mm instead. The else branch it falls to already exists
here, so unlike the SMP side this is the whole fix.
arch/alpha/kernel/smp.c is not built with CONFIG_SMP=n, so this is how
the same defect reaches a uniprocessor, where the flusher kworker
necessarily shares the only CPU with the writer.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/include/asm/tlbflush.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/alpha/include/asm/tlbflush.h b/arch/alpha/include/asm/tlbflush.h
index 0c8529997f54..9ae1902faf8e 100644
--- a/arch/alpha/include/asm/tlbflush.h
+++ b/arch/alpha/include/asm/tlbflush.h
@@ -87,7 +87,8 @@ flush_tlb_page(struct vm_area_struct *vma, unsigned long addr)
 {
 	struct mm_struct *mm = vma->vm_mm;
 
-	if (mm == current->active_mm)
+	/* A targeted tbi() needs a thread of MM to be current.  */
+	if (mm == current->mm)
 		flush_tlb_current_page(mm, vma, addr);
 	else
 		flush_tlb_other(mm);
-- 
2.43.0

[PATCH v3 6/7] alpha: invalidate the local context in flush_tlb_mm()

[Magnus Lindholm]

flush_tlb_mm() has the same caller-CPU omission that was fixed in
flush_tlb_page(): when the target mm is not the calling CPU's active_mm
nothing happens locally, and smp_call_function() handles only the other
CPUs, so this CPU may later reuse the old ASN together with the
translations it still holds.

The active_mm test itself is left alone here. That path calls
flush_tlb_current(), which loads a fresh context through
__load_new_mm_context() rather than issuing a targeted tbi() against
whatever ASN happens to be loaded, so it does not depend on which context
is current.

The uniprocessor implementation in asm/tlbflush.h already has the missing
branch. Counted over a fork-heavy run, flush_tlb_mm() was entered with the
mm not this CPU's active_mm 2934 times, and 632 times while otherwise idle.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index a5a42ae4a7d8..988e397b0b8a 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -649,6 +649,9 @@ flush_tlb_mm(struct mm_struct *mm)
 			preempt_enable();
 			return;
 		}
+	} else {
+		/* smp_call_function() does not call back into this CPU.  */
+		flush_tlb_other(mm);
 	}
 
 	smp_call_function(ipi_flush_tlb_mm, mm, 1);
-- 
2.43.0

Re: [PATCH v3 6/7] alpha: invalidate the local context in flush_tlb_mm()

[Matt Turner]

On Wed, Sep 23, 2026 at 09:47:49AM +0200, Magnus Lindholm wrote:
> The uniprocessor implementation in asm/tlbflush.h already has the missing
> branch. Counted over a fork-heavy run, flush_tlb_mm() was entered with the
> mm not this CPU's active_mm 2934 times, and 632 times while otherwise idle.

With the hugetlb series applied this path gets busier: every hugetlb
flush there goes through flush_tlb_range(), so unmapping or migrating a
hugetlb folio from another context ends up here with a foreign mm.
That might also give you the reproducer this patch lacks: move_pages()
on another process's hugetlb mapping, with the caller pinned to the
CPU the target last ran on.

Reviewed-by: Matt Turner <mattst88@gmail.com>

[PATCH v3 7/7] alpha: invalidate the local context in flush_icache_user_page()

[Magnus Lindholm]

flush_icache_user_page() has the same caller-CPU omission that the
previous patch fixed in flush_tlb_mm(): when the target mm is not the
calling CPU's active_mm nothing happens locally, and smp_call_function()
handles only the other CPUs, so this CPU may later reuse the old ASN
together with the translations it still holds.

This matters here in particular because the function exists for
operating on another process's mappings: the comment above it describes
setting breakpoints through ptrace, and access_remote_vm() reaches it
through copy_to_user_page(). The calling CPU is therefore often running
something other than the target mm.

As in flush_tlb_mm(), the uniprocessor implementation in
asm/cacheflush.h already has the missing case.

No imb() is needed, here or in ipi_flush_icache_page(). Alpha's
user-space I-cache flush works by allocating a new ASN rather than by
invalidating the I-cache: the entries stay, but they are tagged with the
old ASN and can no longer match. An imb() is only required when the ASN
space wraps and numbers are reused, and __get_new_mm_context() already
does one in that case.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/kernel/smp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
index 988e397b0b8a..e21bc3920bec 100644
--- a/arch/alpha/kernel/smp.c
+++ b/arch/alpha/kernel/smp.c
@@ -758,6 +758,9 @@ flush_icache_user_page(struct vm_area_struct *vma, struct page *page,
 			preempt_enable();
 			return;
 		}
+	} else {
+		/* smp_call_function() does not call back into this CPU.  */
+		flush_tlb_other(mm);
 	}
 
 	smp_call_function(ipi_flush_icache_page, mm, 1);
-- 
2.43.0