[PATCH 1/1] arch/mm/fault: fix major fault accounting when retrying under per-VMA lock

[Suren Baghdasaryan]

A test [1] in Android test suite started failing after [2] was merged.
It turns out that after handling a major fault under per-VMA lock, the
process major fault counter does not register that fault as major.
Before [2] read faults would be done under mmap_lock, in which case
FAULT_FLAG_TRIED flag is set before retrying. That in turn causes
mm_account_fault() to account the fault as major once retry completes.
With per-VMA locks we often retry because a fault can't be handled
without locking the whole mm using mmap_lock. Therefore such retries
do not set FAULT_FLAG_TRIED flag. This logic does not work after [2]
because we can now handle read major faults under per-VMA lock and
upon retry the fact there was a major fault gets lost. Fix this by
setting FAULT_FLAG_TRIED after retrying under per-VMA lock if
VM_FAULT_MAJOR was returned. Ideally we would use an additional
VM_FAULT bit to indicate the reason for the retry (could not handle
under per-VMA lock vs other reason) but this simpler solution seems
to work, so keeping it simple.

[1] https://cs.android.com/android/platform/superproject/+/master:test/vts-testcase/kernel/api/drop_caches_prop/drop_caches_test.cpp
[2] https://lore.kernel.org/all/20231006195318.4087158-6-willy@xxxxxxxxxxxxx/

Fixes: 12214eba1992 ("mm: handle read faults under the VMA lock")
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
 arch/arm64/mm/fault.c   | 2 ++
 arch/powerpc/mm/fault.c | 2 ++
 arch/riscv/mm/fault.c   | 2 ++
 arch/s390/mm/fault.c    | 3 +++
 arch/x86/mm/fault.c     | 2 ++
 5 files changed, 11 insertions(+)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 460d799e1296..55f6455a8284 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -607,6 +607,8 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
 		goto done;
 	}
 	count_vm_vma_lock_event(VMA_LOCK_RETRY);
+	if (fault & VM_FAULT_MAJOR)
+		mm_flags |= FAULT_FLAG_TRIED;
 
 	/* Quick path to respond to signals */
 	if (fault_signal_pending(fault, regs)) {
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index 9e49ede2bc1c..53335ae21a40 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -497,6 +497,8 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
 		goto done;
 	}
 	count_vm_vma_lock_event(VMA_LOCK_RETRY);
+	if (fault & VM_FAULT_MAJOR)
+		flags |= FAULT_FLAG_TRIED;
 
 	if (fault_signal_pending(fault, regs))
 		return user_mode(regs) ? 0 : SIGBUS;
diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
index 90d4ba36d1d0..081339ddf47e 100644
--- a/arch/riscv/mm/fault.c
+++ b/arch/riscv/mm/fault.c
@@ -304,6 +304,8 @@ void handle_page_fault(struct pt_regs *regs)
 		goto done;
 	}
 	count_vm_vma_lock_event(VMA_LOCK_RETRY);
+	if (fault & VM_FAULT_MAJOR)
+		flags |= FAULT_FLAG_TRIED;
 
 	if (fault_signal_pending(fault, regs)) {
 		if (!user_mode(regs))
diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 249aefcf7c4e..ab4098886e56 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -337,6 +337,9 @@ static void do_exception(struct pt_regs *regs, int access)
 		return;
 	}
 	count_vm_vma_lock_event(VMA_LOCK_RETRY);
+	if (fault & VM_FAULT_MAJOR)
+		flags |= FAULT_FLAG_TRIED;
+
 	/* Quick path to respond to signals */
 	if (fault_signal_pending(fault, regs)) {
 		if (!user_mode(regs))
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index ab778eac1952..679b09cfe241 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1370,6 +1370,8 @@ void do_user_addr_fault(struct pt_regs *regs,
 		goto done;
 	}
 	count_vm_vma_lock_event(VMA_LOCK_RETRY);
+	if (fault & VM_FAULT_MAJOR)
+		flags |= FAULT_FLAG_TRIED;
 
 	/* Quick path to respond to signals */
 	if (fault_signal_pending(fault, regs)) {
-- 
2.43.0.472.g3155946c3a-goog