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Date: Thu, 4 Jul 2019 12:14:07 -0700
From: Matthew Wilcox <willy@...radead.org>
To: Jan Kara <jack@...e.cz>
Cc: Dan Williams <dan.j.williams@...el.com>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
Boaz Harrosh <openosd@...il.com>,
stable <stable@...r.kernel.org>,
Robert Barror <robert.barror@...el.com>,
Seema Pandit <seema.pandit@...el.com>,
linux-nvdimm <linux-nvdimm@...ts.01.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dax: Fix missed PMD wakeups
On Thu, Jul 04, 2019 at 06:54:50PM +0200, Jan Kara wrote:
> On Wed 03-07-19 20:27:28, Matthew Wilcox wrote:
> > So I think we're good for all current users.
>
> Agreed but it is an ugly trap. As I already said, I'd rather pay the
> unnecessary cost of waiting for pte entry and have an easy to understand
> interface. If we ever have a real world use case that would care for this
> optimization, we will need to refactor functions to make this possible and
> still keep the interfaces sane. For example get_unlocked_entry() could
> return special "error code" indicating that there's no entry with matching
> order in xarray but there's a conflict with it. That would be much less
> error-prone interface.
This is an internal interface. I think it's already a pretty gnarly
interface to use by definition -- it's going to sleep and might return
almost anything. There's not much scope for returning an error indicator
either; value entries occupy half of the range (all odd numbers between 1
and ULONG_MAX inclusive), plus NULL. We could use an internal entry, but
I don't think that makes the interface any easier to use than returning
a locked entry.
I think this iteration of the patch makes it a little clearer. What do you
think?
diff --git a/fs/dax.c b/fs/dax.c
index 2e48c7ebb973..398b601259f9 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -198,8 +198,11 @@ static void dax_wake_entry(struct xa_state *xas, void *entry, bool wake_all)
* if it did.
*
* Must be called with the i_pages lock held.
+ *
+ * If order is non-zero, then a locked smaller entry (eg a PTE entry)
+ * may be returned.
*/
-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;
@@ -211,7 +214,8 @@ static void *get_unlocked_entry(struct xa_state *xas)
for (;;) {
entry = xas_find_conflict(xas);
if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
- !dax_is_locked(entry))
+ !dax_is_locked(entry) ||
+ dax_entry_order(entry) < order)
return entry;
wq = dax_entry_waitqueue(xas, entry, &ewait.key);
@@ -253,8 +257,12 @@ 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 we were the only waiter woken, wake the next one.
+ * Do not wake anybody if the entry is locked; that indicates
+ * we weren't woken.
+ */
+ if (entry && !dax_is_locked(entry))
dax_wake_entry(xas, entry, false);
}
@@ -461,7 +469,7 @@ 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? */
@@ -469,7 +477,7 @@ 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 (!xa_is_value(entry)) {
@@ -477,7 +485,7 @@ static void *grab_mapping_entry(struct xa_state *xas,
goto out_unlock;
}
- if (size_flag & DAX_PMD) {
+ if (order == PMD_ORDER) {
if (dax_is_pte_entry(entry)) {
put_unlocked_entry(xas, entry);
goto fallback;
@@ -523,7 +531,10 @@ 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 +605,7 @@ 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 +632,7 @@ 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 +860,7 @@ 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, dax_entry_order(entry));
/* Entry got punched out / reallocated? */
if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
@@ -861,6 +872,9 @@ static int dax_writeback_one(struct xa_state *xas, struct dax_device *dax_dev,
*/
if (dax_to_pfn(old_entry) != dax_to_pfn(entry))
goto put_unlocked;
+ /* Did a PMD entry get split? */
+ if (dax_is_locked(entry))
+ goto put_unlocked;
if (WARN_ON_ONCE(dax_is_empty_entry(entry) ||
dax_is_zero_entry(entry))) {
ret = -EIO;
@@ -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;
@@ -1659,7 +1673,7 @@ 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)) ||
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