Re: [PATCH] dax: make sure inodes are flushed before destroy cache
On Mon, Feb 14, 2022 at 12:09:54PM -0800, Dan Williams wrote: > On Mon, Feb 14, 2022 at 9:59 AM Ira Weiny wrote: > > > > On Fri, Feb 11, 2022 at 11:11:11PM -0800, Tong Zhang wrote: > > > A bug can be triggered by following command > > > > > > $ modprobe nd_pmem && modprobe -r nd_pmem > > > > > > [ 10.060014] BUG dax_cache (Not tainted): Objects remaining in > > > dax_cache on __kmem_cache_shutdown() > > > [ 10.060938] Slab 0x85b729ac objects=9 used=1 > > > fp=0x4f5ae469 flags=0x2010200(slab|head|node) > > > [ 10.062433] Call Trace: > > > [ 10.062673] dump_stack_lvl+0x34/0x44 > > > [ 10.062865] slab_err+0x90/0xd0 > > > [ 10.063619] __kmem_cache_shutdown+0x13b/0x2f0 > > > [ 10.063848] kmem_cache_destroy+0x4a/0x110 > > > [ 10.064058] __x64_sys_delete_module+0x265/0x300 > > > > > > This is caused by dax_fs_exit() not flushing inodes before destroy cache. > > > To fix this issue, call rcu_barrier() before destroy cache. > > > > I don't doubt that this fixes the bug. However, I can't help but think > > this is > > hiding a bug, or perhaps a missing step, in the kmem_cache layer? As far > > as I > > can see dax does not call call_rcu() and only uses srcu not rcu? I was > > tempted > > to suggest srcu_barrier() but dax does not call call_srcu() either. > > This rcu_barrier() is associated with the call_rcu() in destroy_inode(). Ok yea. > > While kern_unmount() does a full sycnrhonize_rcu() after clearing > ->mnt_ns. Any pending destroy_inode() callbacks need to be flushed > before the kmem_cache is destroyed. > > > So I'm not clear about what is really going on and why this fixes it. I > > know > > that dax is not using srcu is a standard way so perhaps this helps in a way > > I > > don't quite grok? If so perhaps a comment here would be in order? > > Looks like a common pattern I missed that all filesystem exit paths implement. I think a comment would be in order, especially since since it looks like every other FS has one: fs/ext4/super.c: ... /* * Make sure all delayed rcu free inodes are flushed before we * destroy cache. */ rcu_barrier(); ... Anyway ok. Reviewed-by: Ira Weiny Thanks for looking Dan, Ira
Re: [PATCH] dax: make sure inodes are flushed before destroy cache
On Mon, Feb 14, 2022 at 9:59 AM Ira Weiny wrote: > > On Fri, Feb 11, 2022 at 11:11:11PM -0800, Tong Zhang wrote: > > A bug can be triggered by following command > > > > $ modprobe nd_pmem && modprobe -r nd_pmem > > > > [ 10.060014] BUG dax_cache (Not tainted): Objects remaining in dax_cache > > on __kmem_cache_shutdown() > > [ 10.060938] Slab 0x85b729ac objects=9 used=1 > > fp=0x4f5ae469 flags=0x2010200(slab|head|node) > > [ 10.062433] Call Trace: > > [ 10.062673] dump_stack_lvl+0x34/0x44 > > [ 10.062865] slab_err+0x90/0xd0 > > [ 10.063619] __kmem_cache_shutdown+0x13b/0x2f0 > > [ 10.063848] kmem_cache_destroy+0x4a/0x110 > > [ 10.064058] __x64_sys_delete_module+0x265/0x300 > > > > This is caused by dax_fs_exit() not flushing inodes before destroy cache. > > To fix this issue, call rcu_barrier() before destroy cache. > > I don't doubt that this fixes the bug. However, I can't help but think this > is > hiding a bug, or perhaps a missing step, in the kmem_cache layer? As far as I > can see dax does not call call_rcu() and only uses srcu not rcu? I was > tempted > to suggest srcu_barrier() but dax does not call call_srcu() either. This rcu_barrier() is associated with the call_rcu() in destroy_inode(). While kern_unmount() does a full sycnrhonize_rcu() after clearing ->mnt_ns. Any pending destroy_inode() callbacks need to be flushed before the kmem_cache is destroyed. > So I'm not clear about what is really going on and why this fixes it. I know > that dax is not using srcu is a standard way so perhaps this helps in a way I > don't quite grok? If so perhaps a comment here would be in order? Looks like a common pattern I missed that all filesystem exit paths implement.
Re: [PATCH] dax: make sure inodes are flushed before destroy cache
On Fri, Feb 11, 2022 at 11:11:11PM -0800, Tong Zhang wrote:
> A bug can be triggered by following command
>
> $ modprobe nd_pmem && modprobe -r nd_pmem
>
> [ 10.060014] BUG dax_cache (Not tainted): Objects remaining in dax_cache on
> __kmem_cache_shutdown()
> [ 10.060938] Slab 0x85b729ac objects=9 used=1 fp=0x4f5ae469
> flags=0x2010200(slab|head|node)
> [ 10.062433] Call Trace:
> [ 10.062673] dump_stack_lvl+0x34/0x44
> [ 10.062865] slab_err+0x90/0xd0
> [ 10.063619] __kmem_cache_shutdown+0x13b/0x2f0
> [ 10.063848] kmem_cache_destroy+0x4a/0x110
> [ 10.064058] __x64_sys_delete_module+0x265/0x300
>
> This is caused by dax_fs_exit() not flushing inodes before destroy cache.
> To fix this issue, call rcu_barrier() before destroy cache.
I don't doubt that this fixes the bug. However, I can't help but think this is
hiding a bug, or perhaps a missing step, in the kmem_cache layer? As far as I
can see dax does not call call_rcu() and only uses srcu not rcu? I was tempted
to suggest srcu_barrier() but dax does not call call_srcu() either.
So I'm not clear about what is really going on and why this fixes it. I know
that dax is not using srcu is a standard way so perhaps this helps in a way I
don't quite grok? If so perhaps a comment here would be in order?
Ira
>
> Signed-off-by: Tong Zhang
> ---
> drivers/dax/super.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/dax/super.c b/drivers/dax/super.c
> index e3029389d809..6bd565fe2e63 100644
> --- a/drivers/dax/super.c
> +++ b/drivers/dax/super.c
> @@ -476,6 +476,7 @@ static int dax_fs_init(void)
> static void dax_fs_exit(void)
> {
> kern_unmount(dax_mnt);
> + rcu_barrier();
> kmem_cache_destroy(dax_cache);
> }
>
> --
> 2.25.1
>
>
Re: [PATCH] dax: make sure inodes are flushed before destroy cache
Looks good, Reviewed-by: Christoph Hellwig
[PATCH] dax: make sure inodes are flushed before destroy cache
A bug can be triggered by following command
$ modprobe nd_pmem && modprobe -r nd_pmem
[ 10.060014] BUG dax_cache (Not tainted): Objects remaining in dax_cache on
__kmem_cache_shutdown()
[ 10.060938] Slab 0x85b729ac objects=9 used=1 fp=0x4f5ae469
flags=0x2010200(slab|head|node)
[ 10.062433] Call Trace:
[ 10.062673] dump_stack_lvl+0x34/0x44
[ 10.062865] slab_err+0x90/0xd0
[ 10.063619] __kmem_cache_shutdown+0x13b/0x2f0
[ 10.063848] kmem_cache_destroy+0x4a/0x110
[ 10.064058] __x64_sys_delete_module+0x265/0x300
This is caused by dax_fs_exit() not flushing inodes before destroy cache.
To fix this issue, call rcu_barrier() before destroy cache.
Signed-off-by: Tong Zhang
---
drivers/dax/super.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index e3029389d809..6bd565fe2e63 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -476,6 +476,7 @@ static int dax_fs_init(void)
static void dax_fs_exit(void)
{
kern_unmount(dax_mnt);
+ rcu_barrier();
kmem_cache_destroy(dax_cache);
}
--
2.25.1
