Re: [PATCH] fs: Kill DCACHE_DONTCACHE dentry even if DCACHE_REFERENCED is set

[Dave Chinner]

On Fri, Aug 28, 2020 at 05:04:14PM +0800, Li, Hao wrote:
> On 2020/8/28 8:35, Dave Chinner wrote:
> > On Thu, Aug 27, 2020 at 05:58:07PM +0800, Li, Hao wrote:
> >> On 2020/8/27 14:37, Dave Chinner wrote:
> >>> On Fri, Aug 21, 2020 at 09:59:53AM +0800, Hao Li wrote:
> >>>> Currently, DCACHE_REFERENCED prevents the dentry with DCACHE_DONTCACHE
> >>>> set from being killed, so the corresponding inode can't be evicted. If
> >>>> the DAX policy of an inode is changed, we can't make policy changing
> >>>> take effects unless dropping caches manually.
> >>>>
> >>>> This patch fixes this problem and flushes the inode to disk to prepare
> >>>> for evicting it.
> >>>>
> >>>> Signed-off-by: Hao Li <lihao2018.fnst@xxxxxxxxxxxxxx>
> >>>> ---
> >>>>  fs/dcache.c | 3 ++-
> >>>>  fs/inode.c  | 2 +-
> >>>>  2 files changed, 3 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/fs/dcache.c b/fs/dcache.c
> >>>> index ea0485861d93..486c7409dc82 100644
> >>>> --- a/fs/dcache.c
> >>>> +++ b/fs/dcache.c
> >>>> @@ -796,7 +796,8 @@ static inline bool fast_dput(struct dentry *dentry)
> >>>>  	 */
> >>>>  	smp_rmb();
> >>>>  	d_flags = READ_ONCE(dentry->d_flags);
> >>>> -	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_DISCONNECTED;
> >>>> +	d_flags &= DCACHE_REFERENCED | DCACHE_LRU_LIST | DCACHE_DISCONNECTED
> >>>> +			| DCACHE_DONTCACHE;
> >>> Seems reasonable, but you need to update the comment above as to
> >>> how this flag fits into this code....
> >> Yes. I will change it. Thanks.
> >>
> >>>>  	/* Nothing to do? Dropping the reference was all we needed? */
> >>>>  	if (d_flags == (DCACHE_REFERENCED | DCACHE_LRU_LIST) && !d_unhashed(dentry))
> >>>> diff --git a/fs/inode.c b/fs/inode.c
> >>>> index 72c4c347afb7..5218a8aebd7f 100644
> >>>> --- a/fs/inode.c
> >>>> +++ b/fs/inode.c
> >>>> @@ -1632,7 +1632,7 @@ static void iput_final(struct inode *inode)
> >>>>  	}
> >>>>  
> >>>>  	state = inode->i_state;
> >>>> -	if (!drop) {
> >>>> +	if (!drop || (drop && (inode->i_state & I_DONTCACHE))) {
> >>>>  		WRITE_ONCE(inode->i_state, state | I_WILL_FREE);
> >>>>  		spin_unlock(&inode->i_lock);
> >>> What's this supposed to do? We'll only get here with drop set if the
> >>> filesystem is mounting or unmounting.
> >> The variable drop will also be set to True if I_DONTCACHE is set on
> >> inode->i_state.
> >> Although mounting/unmounting will set the drop variable, it won't set
> >> I_DONTCACHE if I understand correctly. As a result,
> >> drop && (inode->i_state & I_DONTCACHE) will filter out mounting/unmounting.
> > So what does this have to do with changing the way the dcache
> > treats DCACHE_DONTCACHE?
> After changing the way the dcache treats DCACHE_DONTCACHE, the dentry with
> DCACHE_DONTCACHE set will be killed unconditionally even if
> DCACHE_REFERENCED is set, and its inode will be processed by iput_final().
> This inode has I_DONTCACHE flag, so op->drop_inode() will return true,
> and the inode will be evicted _directly_ even though it has dirty pages.

Yes. But this doesn't rely on DCACHE_DONTCACHE behaviour. Inodes
that are looked up and cached by the filesystem without going
through dentry cache pathwalks can have I_DONTCACHE set and then get
evicted...

i.e. we can get I_DONTCACHE set on inodes that do not have dentries
attached to them. That's the original functionality that got pulled
up from XFS - internal iteration of inodes, either via quotacheck or
bulkstat....

> I think the kernel will run into error state because it doesn't writeback
> dirty pages of this inode before evicting. This is why I write back this
> inode here.
> 
> According to my test, if I revert the second hunk of this patch, kernel
> will hang after running the following command:
> echo 123 > test.txt && xfs_io -c "chattr +x" test.txt
> 
> The backtrace is:
> 
> xfs_fs_destroy_inode+0x204/0x24d
> destroy_inode+0x3b/0x65
> evict+0x150/0x1aa
> iput+0x117/0x19a
> dentry_unlink_inode+0x12b/0x12f
> __dentry_kill+0xee/0x211
> dentry_kill+0x112/0x22f
> dput+0x79/0x86
> __fput+0x200/0x23f
> ____fput+0x25/0x28
> task_work_run+0x144/0x177
> do_exit+0x4fb/0xb94
> 
> This backtrace is printed with an ASSERT(0) statement in xfs_fs_destroy_inode().

Sure, that's your messenger. That doesn't mean the bug can't be
triggered by other means.

> > Also, if I_DONTCACHE is set, but the inode has also been unlinked or
> > is unhashed, should we be writing it back? i.e. it might have been
> > dropped for a different reason to I_DONTCACHE....
> This is a problem I didn't realize. You are right. If the inode has been
> unlinked/unhashed and the I_DONTCACHE is also set, the appended condition
> will lead to unnecessary writeback.
> 
> I think I need to handle the inode writeback more carefully. Maybe I can
> revert the second hunk and remove I_DONTCACHE from generic_drop_inode()
> and then change
> 
> if (!drop && (sb->s_flags & SB_ACTIVE))
> 
> to
> 
> if (!drop && !(inode->i_state & I_DONTCACHE) && (sb->s_flags & SB_ACTIVE))
> 
> This approach would be more suitable.

Yup, that's pretty much what I was thinking, but as a standalone
patch and preceding the DCACHE_DONTCACHE behaviour change. Thanks! :)

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx