This is V2 of the patch. Posted V1 here.
https://lore.kernel.org/linux-fsdevel/20210416173524.ga1379...@redhat.com/
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 <s...@redhat.com>
Signed-off-by: Vivek Goyal <vgo...@redhat.com>
---
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);
