[PATCH v6 2/3] mm: memory: Add access_remote_vm_locked variant

[Sumit Semwal]

This allows accessing a remote vm while the mmap_lock is already
held by the caller.

While adding support for anonymous vma naming, show_map_vma()
needs to access the remote vm to get the name of the anonymous vma.
Since show_map_vma() already holds the mmap_lock, so this _locked
variant was required.

Signed-off-by: Sumit Semwal <sumit.sem...@linaro.org>
---
 include/linux/mm.h |  2 ++
 mm/memory.c        | 49 ++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ca6e6a81576b..e9212c0bb5ac 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1708,6 +1708,8 @@ extern int access_remote_vm(struct mm_struct *mm, 
unsigned long addr,
                void *buf, int len, unsigned int gup_flags);
 extern int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
                unsigned long addr, void *buf, int len, unsigned int gup_flags);
+extern int access_remote_vm_locked(struct mm_struct *mm, unsigned long addr,
+                                  void *buf, int len, unsigned int gup_flags);
 
 long get_user_pages_remote(struct mm_struct *mm,
                            unsigned long start, unsigned long nr_pages,
diff --git a/mm/memory.c b/mm/memory.c
index 602f4283122f..207be99390e9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4726,17 +4726,17 @@ EXPORT_SYMBOL_GPL(generic_access_phys);
 /*
  * Access another process' address space as given in mm.  If non-NULL, use the
  * given task for page fault accounting.
+ * This variant assumes that the mmap_lock is already held by the caller, so
+ * doesn't take the mmap_lock.
  */
-int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
-               unsigned long addr, void *buf, int len, unsigned int gup_flags)
+int __access_remote_vm_locked(struct task_struct *tsk, struct mm_struct *mm,
+                             unsigned long addr, void *buf, int len,
+                             unsigned int gup_flags)
 {
        struct vm_area_struct *vma;
        void *old_buf = buf;
        int write = gup_flags & FOLL_WRITE;
 
-       if (mmap_read_lock_killable(mm))
-               return 0;
-
        /* ignore errors, just check how much was successfully transferred */
        while (len) {
                int bytes, ret, offset;
@@ -4785,9 +4785,46 @@ int __access_remote_vm(struct task_struct *tsk, struct 
mm_struct *mm,
                buf += bytes;
                addr += bytes;
        }
+       return buf - old_buf;
+}
+
+/*
+ * Access another process' address space as given in mm.  If non-NULL, use the
+ * given task for page fault accounting.
+ */
+int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
+                      unsigned long addr, void *buf, int len, unsigned int 
gup_flags)
+{
+       int ret;
+
+       if (mmap_read_lock_killable(mm))
+               return 0;
+
+       ret = __access_remote_vm_locked(tsk, mm, addr, buf, len, gup_flags);
        mmap_read_unlock(mm);
 
-       return buf - old_buf;
+       return ret;
+}
+
+/**
+ * access_remote_vm_locked - access another process' address space, without
+ * taking the mmap_lock. This allows nested calls from callers that already 
have
+ * taken the lock.
+ *
+ * @mm:                the mm_struct of the target address space
+ * @addr:      start address to access
+ * @buf:       source or destination buffer
+ * @len:       number of bytes to transfer
+ * @gup_flags: flags modifying lookup behaviour
+ *
+ * The caller must hold a reference on @mm, as well as hold the mmap_lock
+ *
+ * Return: number of bytes copied from source to destination.
+ */
+int access_remote_vm_locked(struct mm_struct *mm, unsigned long addr, void 
*buf,
+                           int len, unsigned int gup_flags)
+{
+       return __access_remote_vm_locked(NULL, mm, addr, buf, len, gup_flags);
 }
 
 /**
-- 
2.28.0