linux-kernel - Re: [RFC PATCH v5 07/10] ovl: cache dirty overlayfs' inode

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Date:   Thu, 7 Oct 2021 20:04:30 +0800
From:   Chengguang Xu <cgxu519@....com>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Chengguang Xu <cgxu519@...ernel.net>
Cc:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v5 07/10] ovl: cache dirty overlayfs' inode

在 2021/10/7 19:09, Miklos Szeredi 写道:
> On Thu, 23 Sept 2021 at 15:08, Chengguang Xu <cgxu519@...ernel.net> wrote:
>> Now drop overlayfs' inode will sync dirty data,
>> so we change to only drop clean inode.
>>
>> The purpose of doing this is to keep compatible
>> behavior with before because without this change
>> dropping overlayfs inode will not trigger syncing
>> of underlying dirty inode.
>>
>> Signed-off-by: Chengguang Xu <cgxu519@...ernel.net>
>> ---
>>   fs/overlayfs/super.c | 16 +++++++++++++++-
>>   1 file changed, 15 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
>> index cddae3ca2fa5..bf4000eb9be8 100644
>> --- a/fs/overlayfs/super.c
>> +++ b/fs/overlayfs/super.c
>> @@ -441,11 +441,25 @@ static int ovl_write_inode(struct inode *inode,
>>          return ret;
>>   }
>>
>> +/*
>> + * In iput_final(), clean inode will drop directly and dirty inode will
>> + * keep in the cache until write back to sync dirty data then add to lru
>> + * list to wait reclaim.
>> + */
>> +static int ovl_drop_inode(struct inode *inode)
>> +{
>> +       struct inode *upper = ovl_inode_upper(inode);
>> +
>> +       if (!upper || !(inode->i_state & I_DIRTY_ALL))
> Could we check upper dirtyness here? That would give a more precise result.

We keep tracking mmapped-file(shared mode) by explicitely marking 
overlay inode dirty,

so if we drop overlay inode by checking upper dirtyness, we may lose 
control on those mmapped upper inodes.

>
> Alternatively don't set .drop_inode (i.e. use generic_drop_inode())
> and set I_DONTCACHE on overlay inodes.  That would cause the upper
> inode to be always written back before eviction.
>
> The latter would result in simpler logic, and I think performance-wise
> it wouldn't matter.  But I may be missing something.

I think we may seperate mmapped-file(shared) inode and other inode by

clear/set I_DONTCACHE flag on overlay inode if you prefer this approach.


Thanks,

Chengguang