linux-kernel - [PATCH v6 2/3] mm: memory: Add access_remote_vm

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Message-Id: <20200901091901.19779-3-sumit.semwal@linaro.org>
Date:   Tue,  1 Sep 2020 14:49:00 +0530
From:   Sumit Semwal <sumit.semwal@...aro.org>
To:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Jonathan Corbet <corbet@....net>
Cc:     Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Michal Hocko <mhocko@...e.com>,
        Colin Cross <ccross@...gle.com>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michel Lespinasse <walken@...gle.com>,
        Michal Koutný <mkoutny@...e.com>,
        Song Liu <songliubraving@...com>,
        Huang Ying <ying.huang@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        chenqiwu <chenqiwu@...omi.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        John Hubbard <jhubbard@...dia.com>,
        Mike Christie <mchristi@...hat.com>,
        Bart Van Assche <bvanassche@....org>,
        Amit Pundir <amit.pundir@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Adrian Reber <areber@...hat.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Thomas Cedeno <thomascedeno@...gle.com>,
        linux-fsdevel@...r.kernel.org,
        John Stultz <john.stultz@...aro.org>,
        Sumit Semwal <sumit.semwal@...aro.org>
Subject: [PATCH v6 2/3] mm: memory: Add access_remote_vm_locked variant

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.semwal@...aro.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