linux-ext4 - Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL

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Date:   Thu, 13 Apr 2023 09:25:54 +0000
From:   Bernd Schubert <bschubert@....com>
To:     Miklos Szeredi <miklos@...redi.hu>
CC:     "axboe@...nel.dk" <axboe@...nel.dk>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        Dharmendra Singh <dsingh@....com>
Subject: Re: [PATCH 1/2] fs: add FMODE_DIO_PARALLEL_WRITE flag

On 4/13/23 09:40, Miklos Szeredi wrote:
> On Wed, 12 Apr 2023 at 15:42, Bernd Schubert <bschubert@....com> wrote:
>>
>> Miklos, Jens,
>>
>> could we please also set this flag for fuse?
>>
>>
>> Thanks,
>> Bernd
>>
>>
>> fuse: Set FMODE_DIO_PARALLEL_WRITE flag
>>
>> From: Bernd Schubert <bschubert@....com>
>>
>> Fuse can also do parallel DIO writes, if userspace has enabled it.
>>
>> Signed-off-by: Bernd Schubert <bschubert@....com>
>> ---
>>   fs/fuse/file.c |    3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
>> index 875314ee6f59..46e7f1196fd1 100644
>> --- a/fs/fuse/file.c
>> +++ b/fs/fuse/file.c
>> @@ -215,6 +215,9 @@ void fuse_finish_open(struct inode *inode, struct file *file)
>>          }
>>          if ((file->f_mode & FMODE_WRITE) && fc->writeback_cache)
>>                  fuse_link_write_file(file);
>> +
>> +       if (ff->open_flags & FOPEN_PARALLEL_DIRECT_WRITES)
>> +               file->f_mode |= FMODE_DIO_PARALLEL_WRITE;
> 
> fuse_direct_write_iter():
> 
> bool exclusive_lock =
>      !(ff->open_flags & FOPEN_PARALLEL_DIRECT_WRITES) ||
>      iocb->ki_flags & IOCB_APPEND ||
>      fuse_direct_write_extending_i_size(iocb, from);
> 
> If the write is size extending, then it will take the lock exclusive.
> OTOH, I guess that it would be unusual for lots of  size extending
> writes to be done in parallel.
> 
> What would be the effect of giving the  FMODE_DIO_PARALLEL_WRITE hint
> and then still serializing the writes?

It used here

https://lore.kernel.org/io-uring/20230403-wound-roundworm-c1660e059b8c@brauner/T/#m5f86985d6c67dd1d01a977475dab542c338372dd


fuse_finish_open has its own lock, so letting uring handle requests in 
parallel should not hurt? Is this going like

application -> uring does parallel requests -> fuse.ko -> fuse-deamon

So when fuse-deamon signals that it can handle parallel DIO, it is just 
fuse.ko that might need its own lock to extend the file?


Thanks,
Bernd