linux-kernel - Re: [PATCH 1/2] netfs: release the folio lock and put the folio before retrying

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Date:   Mon, 4 Jul 2022 14:58:33 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>, idryomov@...il.com,
        dhowells@...hat.com
Cc:     vshankar@...hat.com, linux-kernel@...r.kernel.org,
        ceph-devel@...r.kernel.org, willy@...radead.org,
        keescook@...omium.org, linux-fsdevel@...r.kernel.org,
        linux-cachefs@...hat.com
Subject: Re: [PATCH 1/2] netfs: release the folio lock and put the folio
 before retrying


On 7/1/22 6:38 PM, Jeff Layton wrote:
> On Fri, 2022-07-01 at 10:29 +0800, xiubli@...hat.com wrote:
>> From: Xiubo Li <xiubli@...hat.com>
>>
>> The lower layer filesystem should always make sure the folio is
>> locked and do the unlock and put the folio in netfs layer.
>>
>> URL: https://tracker.ceph.com/issues/56423
>> Signed-off-by: Xiubo Li <xiubli@...hat.com>
>> ---
>>   fs/netfs/buffered_read.c | 5 ++++-
>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
>> index 42f892c5712e..257fd37c2461 100644
>> --- a/fs/netfs/buffered_read.c
>> +++ b/fs/netfs/buffered_read.c
>> @@ -351,8 +351,11 @@ int netfs_write_begin(struct netfs_inode *ctx,
>>   		ret = ctx->ops->check_write_begin(file, pos, len, folio, _fsdata);
>>   		if (ret < 0) {
>>   			trace_netfs_failure(NULL, NULL, ret, netfs_fail_check_write_begin);
>> -			if (ret == -EAGAIN)
>> +			if (ret == -EAGAIN) {
>> +				folio_unlock(folio);
>> +				folio_put(folio);
>>   				goto retry;
>> +			}
>>   			goto error;
>>   		}
>>   	}
> I don't know here... I think it might be better to just expect that when
> this function returns an error that the folio has already been unlocked.
> Doing it this way will mean that you will lock and unlock the folio a
> second time for no reason.
>
> Maybe something like this instead?
>
> diff --git a/fs/netfs/buffered_read.c b/fs/netfs/buffered_read.c
> index 42f892c5712e..8ae7b0f4c909 100644
> --- a/fs/netfs/buffered_read.c
> +++ b/fs/netfs/buffered_read.c
> @@ -353,7 +353,7 @@ int netfs_write_begin(struct netfs_inode *ctx,
>                          trace_netfs_failure(NULL, NULL, ret, netfs_fail_check_write_begin);
>                          if (ret == -EAGAIN)
>                                  goto retry;
> -                       goto error;
> +                       goto error_unlocked;
>                  }
>          }
>   
> @@ -418,6 +418,7 @@ int netfs_write_begin(struct netfs_inode *ctx,
>   error:
>          folio_unlock(folio);
>          folio_put(folio);
> +error_unlocked:

Should we also put the folio in ceph and afs ? Won't it introduce 
something like use-after-free bug ?

Maybe we should unlock it in ceph and afs and put it in netfs layer.

-- Xiubo



>          _leave(" = %d", ret);
>          return ret;
>   }
>