This is V2 of the patch. Posted V1 here. https://lore.kernel.org/linux-fsdevel/20210416173524.GA1379987@xxxxxxxxxx/ Based on feedback from Dan and Jan, modified the patch to wake up all waiters when dax entry is invalidated. This solves the issues of missed wakeups. I am seeing missed wakeups which ultimately lead to a deadlock when I am using virtiofs with DAX enabled and running "make -j". I had to mount virtiofs as rootfs and also reduce to dax window size to 256M to reproduce the problem consistently. So here is the problem. put_unlocked_entry() wakes up waiters only if entry is not null as well as !dax_is_conflict(entry). But if I call multiple instances of invalidate_inode_pages2() in parallel, then I can run into a situation where there are waiters on this index but nobody will wait these. invalidate_inode_pages2() invalidate_inode_pages2_range() invalidate_exceptional_entry2() dax_invalidate_mapping_entry_sync() __dax_invalidate_entry() { xas_lock_irq(&xas); entry = get_unlocked_entry(&xas, 0); ... ... dax_disassociate_entry(entry, mapping, trunc); xas_store(&xas, NULL); ... ... put_unlocked_entry(&xas, entry); xas_unlock_irq(&xas); } Say a fault in in progress and it has locked entry at offset say "0x1c". Now say three instances of invalidate_inode_pages2() are in progress (A, B, C) and they all try to invalidate entry at offset "0x1c". Given dax entry is locked, all tree instances A, B, C will wait in wait queue. When dax fault finishes, say A is woken up. It will store NULL entry at index "0x1c" and wake up B. When B comes along it will find "entry=0" at page offset 0x1c and it will call put_unlocked_entry(&xas, 0). And this means put_unlocked_entry() will not wake up next waiter, given the current code. And that means C continues to wait and is not woken up. This patch fixes the issue by waking up all waiters when a dax entry has been invalidated. This seems to fix the deadlock I am facing and I can make forward progress. Reported-by: Sergio Lopez <slp@xxxxxxxxxx> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx> --- fs/dax.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) Index: redhat-linux/fs/dax.c =================================================================== --- redhat-linux.orig/fs/dax.c 2021-04-16 14:16:44.332140543 -0400 +++ redhat-linux/fs/dax.c 2021-04-19 11:24:11.465213474 -0400 @@ -264,11 +264,11 @@ static void wait_entry_unlocked(struct x finish_wait(wq, &ewait.wait); } -static void put_unlocked_entry(struct xa_state *xas, void *entry) +static void put_unlocked_entry(struct xa_state *xas, void *entry, bool wake_all) { /* If we were the only waiter woken, wake the next one */ if (entry && !dax_is_conflict(entry)) - dax_wake_entry(xas, entry, false); + dax_wake_entry(xas, entry, wake_all); } /* @@ -622,7 +622,7 @@ struct page *dax_layout_busy_page_range( entry = get_unlocked_entry(&xas, 0); if (entry) page = dax_busy_page(entry); - put_unlocked_entry(&xas, entry); + put_unlocked_entry(&xas, entry, false); if (page) break; if (++scanned % XA_CHECK_SCHED) @@ -664,7 +664,7 @@ static int __dax_invalidate_entry(struct mapping->nrexceptional--; ret = 1; out: - put_unlocked_entry(&xas, entry); + put_unlocked_entry(&xas, entry, true); xas_unlock_irq(&xas); return ret; } @@ -943,7 +943,7 @@ static int dax_writeback_one(struct xa_s return ret; put_unlocked: - put_unlocked_entry(xas, entry); + put_unlocked_entry(xas, entry, false); return ret; } @@ -1684,7 +1684,7 @@ dax_insert_pfn_mkwrite(struct vm_fault * /* Did we race with someone splitting entry or so? */ if (!entry || dax_is_conflict(entry) || (order == 0 && !dax_is_pte_entry(entry))) { - put_unlocked_entry(&xas, entry); + put_unlocked_entry(&xas, entry, false); xas_unlock_irq(&xas); trace_dax_insert_pfn_mkwrite_no_entry(mapping->host, vmf, VM_FAULT_NOPAGE);
