From: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> commit 23c84eb7837514e16d79ed6d849b13745e0ce688 upstream. 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@xxxxxxxxxxxxxxx> Link: http://lkml.kernel.org/r/CAPcyv4hwHpX-MkUEqxwdTj7wCCZCN4RV-L4jsnuwLGyL_UEG4A@xxxxxxxxxxxxxx Fixes: b15cd800682f ("dax: Convert page fault handlers to XArray") Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Tested-by: Dan Williams <dan.j.williams@xxxxxxxxx> Reported-by: Robert Barror <robert.barror@xxxxxxxxx> Reported-by: Seema Pandit <seema.pandit@xxxxxxxxx> Reviewed-by: Jan Kara <jack@xxxxxxx> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/dax.c | 53 +++++++++++++++++++++++++++++++++-------------------- 1 file changed, 33 insertions(+), 20 deletions(-) --- a/fs/dax.c +++ b/fs/dax.c @@ -124,6 +124,15 @@ static int dax_is_empty_entry(void *entr } /* + * 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 */ struct exceptional_entry_key { @@ -195,11 +204,13 @@ static void dax_wake_entry(struct xa_sta * 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 x 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 x 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, * 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 x 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 @@ retry: 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 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 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 && @@ -848,7 +862,7 @@ static int dax_writeback_one(struct xa_s 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))) @@ -1509,7 +1523,7 @@ static vm_fault_t dax_iomap_pmd_fault(st * 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; @@ -1658,11 +1672,10 @@ dax_insert_pfn_mkwrite(struct vm_fault * 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,
