Patch "dax: fix race between colliding PMD & PTE entries" has been added to the 4.11-stable tree

This is a note to let you know that I've just added the patch titled

dax: fix race between colliding PMD & PTE entries

to the 4.11-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dax-fix-race-between-colliding-pmd-pte-entries.patch
and it can be found in the queue-4.11 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ross Zwisler <[email protected]>
Date: Fri, 2 Jun 2017 14:46:37 -0700
Subject: dax: fix race between colliding PMD & PTE entries

From: Ross Zwisler <[email protected]>

commit e2093926a098a8ccf0f1d10f6df8dad452cb28d3 upstream.

We currently have two related PMD vs PTE races in the DAX code. These can both be easily triggered by having two threads reading and writing simultaneously to the same private mapping, with the key being that private mapping reads can be handled with PMDs but private mapping writes are always handled with PTEs so that we can COW.

Here is the first race:

CPU 0 CPU 1

(private mapping write)
__handle_mm_fault()
create_huge_pmd() - FALLBACK
handle_pte_fault()
passes check for pmd_devmap()

(private mapping read) __handle_mm_fault()
create_huge_pmd()
dax_iomap_pmd_fault() inserts PMD

dax_iomap_pte_fault() does a PTE fault, but we already have a DAX PMD
installed in our page tables at this spot.

Here's the second race:

CPU 0 CPU 1

(private mapping read)
__handle_mm_fault()
passes check for pmd_none()
create_huge_pmd()
dax_iomap_pmd_fault() inserts PMD

(private mapping write)
__handle_mm_fault()
create_huge_pmd() - FALLBACK
(private mapping read) __handle_mm_fault()
passes check for pmd_none() create_huge_pmd()

handle_pte_fault()
dax_iomap_pte_fault() inserts PTE
dax_iomap_pmd_fault() inserts PMD,
but we already have a PTE at this spot.

The core of the issue is that while there is isolation between faults to the same range in the DAX fault handlers via our DAX entry locking, there is no isolation between faults in the code in mm/memory.c. This means for instance that this code in __handle_mm_fault() can run:

	if (pmd_none(*vmf.pmd) && transparent_hugepage_enabled(vma)) {
		ret = create_huge_pmd(&vmf);

But by the time we actually get to run the fault handler called by create_huge_pmd(), the PMD is no longer pmd_none() because a racing PTE fault has installed a normal PMD here as a parent. This is the cause of the 2nd race. The first race is similar - there is the following check in handle_pte_fault():

	} else {
		/* See comment in pte_alloc_one_map() */
		if (pmd_devmap(*vmf->pmd) || pmd_trans_unstable(vmf->pmd))
			return 0;

So if a pmd_devmap() PMD (a DAX PMD) has been installed at vmf->pmd, we will bail and retry the fault. This is correct, but there is nothing preventing the PMD from being installed after this check but before we actually get to the DAX PTE fault handlers.

In my testing these races result in the following types of errors:

  BUG: Bad rss-counter state mm:ffff8800a817d280 idx:1 val:1
  BUG: non-zero nr_ptes on freeing mm: 15

Fix this issue by having the DAX fault handlers verify that it is safe to continue their fault after they have taken an entry lock to block other racing faults.

---
 fs/dax.c |   23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

@@ -1129,6 +1129,17 @@ static int dax_iomap_pte_fault(struct vm
 		return dax_fault_return(PTR_ERR(entry));
 
 	/*
+	 * It is possible, particularly with mixed reads & writes to private
+	 * mappings, that we have raced with a PMD fault that overlaps with
+	 * the PTE we need to set up.  If so just return and the fault will be
+	 * retried.
+	 */
+	if (pmd_trans_huge(*vmf->pmd) || pmd_devmap(*vmf->pmd)) {
+		vmf_ret = VM_FAULT_NOPAGE;
+		goto unlock_entry;
+	}
+
+	/*
 	 * Note that we don't bother to use iomap_apply here: DAX required
 	 * the file system block size to be equal the page size, which means
 	 * that we never have to deal with more than a single extent here.

@@ -1363,6 +1374,18 @@ static int dax_iomap_pmd_fault(struct vm
 		goto fallback;
 
 	/*
+	 * It is possible, particularly with mixed reads & writes to private
+	 * mappings, that we have raced with a PTE fault that overlaps with
+	 * the PMD we need to set up.  If so just return and the fault will be
+	 * retried.
+	 */
+	if (!pmd_none(*vmf->pmd) && !pmd_trans_huge(*vmf->pmd) &&
+			!pmd_devmap(*vmf->pmd)) {
+		result = 0;
+		goto unlock_entry;
+	}
+
+	/*
 	 * Note that we don't use iomap_apply here.  We aren't doing I/O, only
 	 * setting up a mapping, so really we're using iomap_begin() as a way
 	 * to look up our filesystem block.

Patches currently in stable-queue which might be from [email protected] are

queue-4.11/mm-avoid-spurious-bad-pmd-warning-messages.patch queue-4.11/dax-fix-race-between-colliding-pmd-pte-entries.patch