On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <vgoyal(a)redhat.com> wrote:
This is V2 of the patch. Posted V1 here.
https://lore.kernel.org/linux-fsdevel/20210416173524.GA1379987@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.
Care to send a formal patch with this commentary moved below the --- line?
One style fixup below...
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(a)redhat.com>
Signed-off-by: Vivek Goyal <vgoyal(a)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);
I'm not a fan of raw true/false arguments because if you read this
line in isolation you need to go read put_unlocked_entry() to recall
what that argument means. So lets add something like:
/**
* enum dax_entry_wake_mode: waitqueue wakeup toggle
* @WAKE_NEXT: entry was not mutated
* @WAKE_ALL: entry was invalidated, or resized
*/
enum dax_entry_wake_mode {
WAKE_NEXT,
WAKE_ALL,
}
...and use that as the arg for dax_wake_entry(). So I'd expect this to
be a 3 patch series, introduce dax_entry_wake_mode for
dax_wake_entry(), introduce the argument for put_unlocked_entry()
without changing the logic, and finally this bug fix. Feel free to add
'Fixes: ac401cc78242 ("dax: New fault locking")' in case you feel this
needs to be backported.