[PATCH][v2] dax: Fix missed wakeup during dax entry invalidation

[PATCH][v2] dax: Fix missed wakeup during dax entry invalidation

[Vivek Goyal]

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.

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@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@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);

Re: [PATCH][v2] dax: Fix missed wakeup during dax entry invalidation

[Dan Williams]

On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <vgoyal@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@redhat.com>
> Signed-off-by: Vivek Goyal <vgoyal@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.

Re: [PATCH][v2] dax: Fix missed wakeup during dax entry invalidation

[Vivek Goyal]

On Mon, Apr 19, 2021 at 12:48:58PM -0700, Dan Williams wrote:
> On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <vgoyal@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@redhat.com>
> > Signed-off-by: Vivek Goyal <vgoyal@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.

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?

Thanks
Vivek

Re: [PATCH][v2] dax: Fix missed wakeup during dax entry invalidation

[Vivek Goyal]

On Mon, Apr 19, 2021 at 04:39:47PM -0400, Vivek Goyal wrote:
> On Mon, Apr 19, 2021 at 12:48:58PM -0700, Dan Williams wrote:
> > On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <vgoyal@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@redhat.com>
> > > Signed-off-by: Vivek Goyal <vgoyal@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.
> 
> 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?

oops, you already mentioned dax_wake_entry(). I read too fast. Sorry for
the noise

Vivek