linux-kernel - Re: [fuse-devel] [PATCH] fuse: avoid deadlock when write fuse inode

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Date:   Sun, 7 Feb 2021 09:47:35 +0800
From:   Huang Jianan <huangjianan@...o.com>
To:     fuse-devel@...ts.sourceforge.net, miklos@...redi.hu
Cc:     guoweichao@...o.com, zhangshiming@...o.com,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [fuse-devel] [PATCH] fuse: avoid deadlock when write fuse inode

friendly ping ... 😁

On 2021/2/2 12:11, Huang Jianan via fuse-devel wrote:
> Hi all,
>
>
> This patch works well in our product, but I am not sure this is the 
> correct
>
> way to solve this problem. I think that the inode->i_count shouldn't be
>
> zero after iput is executed in dentry_unlink_inode, then the inode won't
>
> be writeback. But i haven't found where iget is missing.
>
>
> Thanks,
>
> Jianan
>
> On 2021/2/2 12:08, Huang Jianan wrote:
>> We found the following deadlock situations in low memory scenarios:
>> Thread A                         Thread B
>> - __writeback_single_inode
>>   - fuse_write_inode
>>    - fuse_simple_request
>>     - __fuse_request_send
>>      - request_wait_answer
>>                                   - fuse_dev_splice_read
>>                                    - fuse_copy_fill
>>                                     - __alloc_pages_direct_reclaim
>>                                      - do_shrink_slab
>>                                       - super_cache_scan
>>                                        - shrink_dentry_list
>>                                         - dentry_unlink_inode
>>                                          - iput_final
>>                                           - inode_wait_for_writeback
>>
>> The request and inode processed by Thread A and B are the same, which
>> causes a deadlock. To avoid this, we remove the __GFP_FS flag when
>> allocating memory in fuse_copy_fill, so there will be no memory
>> reclaimation in super_cache_scan.
>>
>> Signed-off-by: Huang Jianan <huangjianan@...o.com>
>> Signed-off-by: Guo Weichao <guoweichao@...o.com>
>> ---
>>   fs/fuse/dev.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
>> index 588f8d1240aa..e580b9d04c25 100644
>> --- a/fs/fuse/dev.c
>> +++ b/fs/fuse/dev.c
>> @@ -721,7 +721,7 @@ static int fuse_copy_fill(struct fuse_copy_state 
>> *cs)
>>               if (cs->nr_segs >= cs->pipe->max_usage)
>>                   return -EIO;
>>   -            page = alloc_page(GFP_HIGHUSER);
>> +            page = alloc_page(GFP_HIGHUSER & ~__GFP_FS);
>>               if (!page)
>>                   return -ENOMEM;
>
>