linux-kernel - [PATCH 5/6] mremap: Check if it's possible to split original vma

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Date:   Tue, 13 Oct 2020 02:34:15 +0100
From:   Dmitry Safonov <dima@...sta.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Dmitry Safonov <dima@...sta.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Geffon <bgeffon@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Minchan Kim <minchan@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH 5/6] mremap: Check if it's possible to split original vma

If original VMA can't be split at the desired address, do_munmap() will
fail and leave both new-copied VMA and old VMA. De-facto it's
MREMAP_DONTUNMAP behaviour, which is unexpected.

Currently, it may fail such way for hugetlbfs and dax device mappings.

Minimize such unpleasant situations to OOM by checking .may_split()
before attempting to create a VMA copy.

Signed-off-by: Dmitry Safonov <dima@...sta.com>
---
 mm/mremap.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 898e9818ba6d..3c4047c23673 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -343,7 +343,7 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 	unsigned long excess = 0;
 	unsigned long hiwater_vm;
 	int split = 0;
-	int err;
+	int err = 0;
 	bool need_rmap_locks;
 
 	/*
@@ -353,6 +353,15 @@ static unsigned long move_vma(struct vm_area_struct *vma,
 	if (mm->map_count >= sysctl_max_map_count - 3)
 		return -ENOMEM;
 
+	if (vma->vm_ops && vma->vm_ops->may_split) {
+		if (vma->vm_start != old_addr)
+			err = vma->vm_ops->may_split(vma, old_addr);
+		if (!err && vma->vm_end != old_addr + old_len)
+			err = vma->vm_ops->may_split(vma, old_addr + old_len);
+		if (err)
+			return err;
+	}
+
 	/*
 	 * Advise KSM to break any KSM pages in the area to be moved:
 	 * it would be confusing if they were to turn up at the new
-- 
2.28.0