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From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>,
	linux-mm@kvack.org, Suren Baghdasaryan <surenb@google.com>
Subject: [PATCH v2 3/6] mm: Handle shared faults under the VMA lock
Date: Fri,  6 Oct 2023 20:53:15 +0100	[thread overview]
Message-ID: <20231006195318.4087158-4-willy@infradead.org> (raw)
In-Reply-To: <20231006195318.4087158-1-willy@infradead.org>

There are many implementations of ->fault and some of them depend on
mmap_lock being held.  All vm_ops that implement ->map_pages() end up
calling filemap_fault(), which I have audited to be sure it does not rely
on mmap_lock.  So (for now) key off ->map_pages existing as a flag to
indicate that it's safe to call ->fault while only holding the vma lock.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/memory.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index cff78c496728..a9b0c135209a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3042,6 +3042,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf)
 	count_vm_event(PGREUSE);
 }
 
+/*
+ * We could add a bitflag somewhere, but for now, we know that all
+ * vm_ops that have a ->map_pages have been audited and don't need
+ * the mmap_lock to be held.
+ */
+static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf)
+{
+	struct vm_area_struct *vma = vmf->vma;
+
+	if (vma->vm_ops->map_pages || !(vmf->flags & FAULT_FLAG_VMA_LOCK))
+		return 0;
+	vma_end_read(vma);
+	return VM_FAULT_RETRY;
+}
+
 static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
 {
 	struct vm_area_struct *vma = vmf->vma;
@@ -4669,10 +4684,9 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
 	vm_fault_t ret, tmp;
 	struct folio *folio;
 
-	if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-		vma_end_read(vma);
-		return VM_FAULT_RETRY;
-	}
+	ret = vmf_can_call_fault(vmf);
+	if (ret)
+		return ret;
 
 	ret = __do_fault(vmf);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
-- 
2.40.1



  parent reply	other threads:[~2023-10-06 19:53 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-10-06 19:53 [PATCH v2 0/6] Handle more faults under the VMA lock Matthew Wilcox (Oracle)
2023-10-06 19:53 ` [PATCH v2 1/6] mm: Make lock_folio_maybe_drop_mmap() VMA lock aware Matthew Wilcox (Oracle)
2023-10-08 21:47   ` Suren Baghdasaryan
2023-10-06 19:53 ` [PATCH v2 2/6] mm: Call wp_page_copy() under the VMA lock Matthew Wilcox (Oracle)
2023-10-08 22:00   ` Suren Baghdasaryan
2023-10-06 19:53 ` Matthew Wilcox (Oracle) [this message]
2023-10-08 22:01   ` [PATCH v2 3/6] mm: Handle shared faults " Suren Baghdasaryan
2023-10-20 13:23   ` kernel test robot
2023-10-06 19:53 ` [PATCH v2 4/6] mm: Handle COW " Matthew Wilcox (Oracle)
2023-10-08 22:05   ` Suren Baghdasaryan
2023-10-20 13:18   ` kernel test robot
2023-10-06 19:53 ` [PATCH v2 5/6] mm: Handle read " Matthew Wilcox (Oracle)
2023-10-08 22:06   ` Suren Baghdasaryan
2023-10-20  9:55   ` kernel test robot
2023-10-06 19:53 ` [PATCH v2 6/6] mm: Handle write faults to RO pages " Matthew Wilcox (Oracle)
2023-10-08 22:07   ` Suren Baghdasaryan

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