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SubjectRe: [PATCH] dax: Fix missed PMD wakeups
On Thu, Jul 11, 2019 at 07:13:50AM -0700, Matthew Wilcox wrote:
> However, the XA_RETRY_ENTRY might be a good choice. It doesn't normally
> appear in an XArray (it may appear if you're looking at a deleted node,
> but since we're holding the lock, we can't see deleted nodes).

Updated patch (also slight updates to changelog and comments)

--- 8< ---

From af8402dacb3998d8ef23677ac35fdb72b236320c Mon Sep 17 00:00:00 2001
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Date: Wed, 3 Jul 2019 23:21:25 -0400
Subject: [PATCH] dax: Fix missed wakeup with PMD faults

RocksDB can hang indefinitely when using a DAX file. This is due to
a bug in the XArray conversion when handling a PMD fault and finding a
PTE entry. We use the wrong index in the hash and end up waiting on
the wrong waitqueue.

There's actually no need to wait; if we find a PTE entry while looking
for a PMD entry, we can return immediately as we know we should fall
back to a PTE fault (which may not conflict with the lock held).

We reuse the XA_RETRY_ENTRY to signal a conflicting entry was found.
This value can never be found in an XArray while holding its lock, so
it does not create an ambiguity.

Cc: stable@vger.kernel.org
Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
fs/dax.c | 53 +++++++++++++++++++++++++++++++++--------------------
1 file changed, 33 insertions(+), 20 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 2e48c7ebb973..7a75031da644 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -123,6 +123,15 @@ static int dax_is_empty_entry(void *entry)
return xa_to_value(entry) & DAX_EMPTY;
}

+/*
+ * true if the entry that was found is of a smaller order than the entry
+ * we were looking for
+ */
+static bool dax_is_conflict(void *entry)
+{
+ return entry == XA_RETRY_ENTRY;
+}
+
/*
* DAX page cache entry locking
*/
@@ -195,11 +204,13 @@ static void dax_wake_entry(struct xa_state *xas, void *entry, bool wake_all)
* Look up entry in page cache, wait for it to become unlocked if it
* is a DAX entry and return it. The caller must subsequently call
* put_unlocked_entry() if it did not lock the entry or dax_unlock_entry()
- * if it did.
+ * if it did. The entry returned may have a larger order than @order.
+ * If @order is larger than the order of the entry found in i_pages, this
+ * function returns a dax_is_conflict entry.
*
* Must be called with the i_pages lock held.
*/
-static void *get_unlocked_entry(struct xa_state *xas)
+static void *get_unlocked_entry(struct xa_state *xas, unsigned int order)
{
void *entry;
struct wait_exceptional_entry_queue ewait;
@@ -210,6 +221,8 @@ static void *get_unlocked_entry(struct xa_state *xas)

for (;;) {
entry = xas_find_conflict(xas);
+ if (dax_entry_order(entry) < order)
+ return XA_RETRY_ENTRY;
if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
!dax_is_locked(entry))
return entry;
@@ -254,7 +267,7 @@ static void wait_entry_unlocked(struct xa_state *xas, void *entry)
static void put_unlocked_entry(struct xa_state *xas, void *entry)
{
/* If we were the only waiter woken, wake the next one */
- if (entry)
+ if (entry && dax_is_conflict(entry))
dax_wake_entry(xas, entry, false);
}

@@ -461,7 +474,7 @@ void dax_unlock_page(struct page *page, dax_entry_t cookie)
* overlap with xarray value entries.
*/
static void *grab_mapping_entry(struct xa_state *xas,
- struct address_space *mapping, unsigned long size_flag)
+ struct address_space *mapping, unsigned int order)
{
unsigned long index = xas->xa_index;
bool pmd_downgrade = false; /* splitting PMD entry into PTE entries? */
@@ -469,20 +482,17 @@ static void *grab_mapping_entry(struct xa_state *xas,

retry:
xas_lock_irq(xas);
- entry = get_unlocked_entry(xas);
+ entry = get_unlocked_entry(xas, order);

if (entry) {
+ if (dax_is_conflict(entry))
+ goto fallback;
if (!xa_is_value(entry)) {
xas_set_err(xas, EIO);
goto out_unlock;
}

- if (size_flag & DAX_PMD) {
- if (dax_is_pte_entry(entry)) {
- put_unlocked_entry(xas, entry);
- goto fallback;
- }
- } else { /* trying to grab a PTE entry */
+ if (order == 0) {
if (dax_is_pmd_entry(entry) &&
(dax_is_zero_entry(entry) ||
dax_is_empty_entry(entry))) {
@@ -523,7 +533,11 @@ static void *grab_mapping_entry(struct xa_state *xas,
if (entry) {
dax_lock_entry(xas, entry);
} else {
- entry = dax_make_entry(pfn_to_pfn_t(0), size_flag | DAX_EMPTY);
+ unsigned long flags = DAX_EMPTY;
+
+ if (order > 0)
+ flags |= DAX_PMD;
+ entry = dax_make_entry(pfn_to_pfn_t(0), flags);
dax_lock_entry(xas, entry);
if (xas_error(xas))
goto out_unlock;
@@ -594,7 +608,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
if (WARN_ON_ONCE(!xa_is_value(entry)))
continue;
if (unlikely(dax_is_locked(entry)))
- entry = get_unlocked_entry(&xas);
+ entry = get_unlocked_entry(&xas, 0);
if (entry)
page = dax_busy_page(entry);
put_unlocked_entry(&xas, entry);
@@ -621,7 +635,7 @@ static int __dax_invalidate_entry(struct address_space *mapping,
void *entry;

xas_lock_irq(&xas);
- entry = get_unlocked_entry(&xas);
+ entry = get_unlocked_entry(&xas, 0);
if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
goto out;
if (!trunc &&
@@ -849,7 +863,7 @@ static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
if (unlikely(dax_is_locked(entry))) {
void *old_entry = entry;

- entry = get_unlocked_entry(xas);
+ entry = get_unlocked_entry(xas, 0);

/* Entry got punched out / reallocated? */
if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
@@ -1510,7 +1524,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
* entry is already in the array, for instance), it will return
* VM_FAULT_FALLBACK.
*/
- entry = grab_mapping_entry(&xas, mapping, DAX_PMD);
+ entry = grab_mapping_entry(&xas, mapping, PMD_ORDER);
if (xa_is_internal(entry)) {
result = xa_to_internal(entry);
goto fallback;
@@ -1659,11 +1673,10 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order)
vm_fault_t ret;

xas_lock_irq(&xas);
- entry = get_unlocked_entry(&xas);
+ entry = get_unlocked_entry(&xas, order);
/* Did we race with someone splitting entry or so? */
- if (!entry ||
- (order == 0 && !dax_is_pte_entry(entry)) ||
- (order == PMD_ORDER && !dax_is_pmd_entry(entry))) {
+ if (!entry || dax_is_conflict(entry) ||
+ (order == 0 && !dax_is_pte_entry(entry))) {
put_unlocked_entry(&xas, entry);
xas_unlock_irq(&xas);
trace_dax_insert_pfn_mkwrite_no_entry(mapping->host, vmf,
--
2.20.1
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