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
===================================================================
--- 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);
On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <[email protected]> wrote:
>
> 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...
>
> 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
> ===================================================================
> --- 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.
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 <[email protected]> wrote:
> > >
> > > 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...
> >
> > >
> > > 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
> > > ===================================================================
> > > --- 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
On Mon, Apr 19, 2021 at 12:48:58PM -0700, Dan Williams wrote:
> On Mon, Apr 19, 2021 at 11:45 AM Vivek Goyal <[email protected]> wrote:
> >
> > 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...
>
> >
> > 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
> > ===================================================================
> > --- 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