dax: Fix missed wakeup during dax entry invalidation

This is V2 of the patch. Posted V1 here.

https://lore.kernel.org/linux-fsdevel/[email protected]/

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 <[email protected]>

Signed-off-by Vivek Goyal <[email protected]>

---
 fs/dax.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Index: redhat-linux/fs/dax.c
===================================================================

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 renamed from fs/dax.c 2021-04-16 14:16:44.332140543 -0400 +2 -2

@@ -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);
 }
 
 /*

Quoted 8 lines · Vivek Goyal

This is V2 of the patch. Posted V1 here.

https://lore.kernel.org/linux-fsdevel/[email protected]/

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...

Quoted 70 lines

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 <[email protected]>

Signed-off-by Vivek Goyal <[email protected]>

---
 fs/dax.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Index: redhat-linux/fs/dax.c
===================================================================

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 renamed from fs/dax.c 2021-04-16 14:16:44.332140543 -0400 +2 -2

@@ -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);
 }
 
 /*

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 struct page *dax_layout_busy_page_range( · line 625

Quoted 5 lines

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 renamed from fs/dax.c 2021-04-16 14:16:44.332140543 -0400 +1 -1

@@ -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.

Quoted 104 lines · Dan Williams

Quoted 8 lines · Vivek Goyal

This is V2 of the patch. Posted V1 here.

https://lore.kernel.org/linux-fsdevel/[email protected]/

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...

Quoted 70 lines

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 <[email protected]>

Signed-off-by Vivek Goyal <[email protected]>

---
 fs/dax.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

Index: redhat-linux/fs/dax.c
===================================================================

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 renamed from fs/dax.c 2021-04-16 14:16:44.332140543 -0400 +2 -2

@@ -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);
 }
 
 /*

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 struct page *dax_layout_busy_page_range( · line 625

Quoted 5 lines

fs/dax.c 2021-04-19 11:24:11.465213474 -0400 renamed from fs/dax.c 2021-04-16 14:16:44.332140543 -0400 +1 -1

@@ -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,
}

Quoted 6 lines

...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.

Hi Dan,

I will make changes as you suggested and post another version.

I am wondering what to do with dax_wake_entry(). It also has a boolean parameter wake_all. Should that be converted as well to make use of enum dax_entry_wake_mode?