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Date:   Fri, 14 Apr 2023 10:54:44 -0700
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Message-ID: <20230414175444.1837474-1-surenb@google.com>
Subject: [PATCH 1/1] mm: do not increment pgfault stats when page fault
 handler retries
From:   Suren Baghdasaryan <surenb@google.com>
To:     akpm@linux-foundation.org
Cc:     willy@infradead.org, hannes@cmpxchg.org, mhocko@suse.com,
        josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com,
        laurent.dufour@fr.ibm.com, michel@lespinasse.org,
        liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz,
        minchan@google.com, dave@stgolabs.net, punit.agrawal@bytedance.com,
        lstoakes@gmail.com, surenb@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
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Precedence: bulk

If the page fault handler requests a retry, we will count the fault
multiple times.  This is a relatively harmless problem as the retry paths
are not often requested, and the only user-visible problem is that the
fault counter will be slightly higher than it should be.  Nevertheless,
userspace only took one fault, and should not see the fact that the
kernel had to retry the fault multiple times.

Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
Patch applies cleanly over linux-next and mm-unstable

 mm/memory.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 1c5b231fe6e3..d88f370eacd1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -5212,17 +5212,16 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 
 	__set_current_state(TASK_RUNNING);
 
-	count_vm_event(PGFAULT);
-	count_memcg_event_mm(vma->vm_mm, PGFAULT);
-
 	ret = sanitize_fault_flags(vma, &flags);
 	if (ret)
-		return ret;
+		goto out;
 
 	if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
 					    flags & FAULT_FLAG_INSTRUCTION,
-					    flags & FAULT_FLAG_REMOTE))
-		return VM_FAULT_SIGSEGV;
+					    flags & FAULT_FLAG_REMOTE)) {
+		ret = VM_FAULT_SIGSEGV;
+		goto out;
+	}
 
 	/*
 	 * Enable the memcg OOM handling for faults triggered in user
@@ -5253,6 +5252,11 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
 	}
 
 	mm_account_fault(regs, address, flags, ret);
+out:
+	if (!(ret & VM_FAULT_RETRY)) {
+		count_vm_event(PGFAULT);
+		count_memcg_event_mm(vma->vm_mm, PGFAULT);
+	}
 
 	return ret;
 }
-- 
2.40.0.634.g4ca3ef3211-goog