The patch titled
Subject: mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page-v2
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page-v2.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page-v2.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Ira Weiny <ira.weiny@xxxxxxxxx>
Subject: mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page-v2
Date: Tue, 25 Oct 2022 15:01:36 -0700
Update the commit message and comment based on additional discussion
Thanks to Matt for pointing out the deadlock potential despite recursive
reads.
Link: https://lkml.kernel.org/r/20221025220136.2366143-1-ira.weiny@xxxxxxxxx
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
--- a/mm/userfaultfd.c~mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page-v2
+++ a/mm/userfaultfd.c
@@ -159,9 +159,19 @@ static int mcopy_atomic_pte(struct mm_st
page_kaddr = kmap_local_page(page);
/*
- * The mmap_lock is held here. Disable page faults to
- * prevent deadlock should copy_from_user() fault. The
- * copy will be retried outside the mmap_lock.
+ * The read mmap_lock is held here. Despite the
+ * mmap_lock being read recursive a deadlock is still
+ * possible if a writer has taken a lock. For example:
+ *
+ * process A thread 1 takes read lock on own mmap_lock
+ * process A thread 2 calls mmap, blocks taking write lock
+ * process B thread 1 takes page fault, read lock on own mmap lock
+ * process B thread 2 calls mmap, blocks taking write lock
+ * process A thread 1 blocks taking read lock on process B
+ * process B thread 1 blocks taking read lock on process A
+ *
+ * Disable page faults to prevent potential deadlock
+ * and retry the copy outside the mmap_lock.
*/
pagefault_disable();
ret = copy_from_user(page_kaddr,
_
Patches currently in -mm which might be from ira.weiny@xxxxxxxxx are
mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page.patch
mm-userfaultfd-replace-kmap-kmap_atomic-with-kmap_local_page-v2.patch
