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)) ||