linux-kernel - Re: [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create

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Date:   Thu, 19 May 2022 19:41:59 +0200
From:   Bernd Schubert <bschubert@....com>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Dharmendra Singh <dharamhans87@...il.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>, linux-fsdevel@...r.kernel.org,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/3] FUSE: Implement atomic lookup + open/create



On 5/19/22 11:39, Miklos Szeredi wrote:
> On Tue, 17 May 2022 at 12:08, Dharmendra Singh <dharamhans87@...il.com> wrote:
>>
>> In FUSE, as of now, uncached lookups are expensive over the wire.
>> E.g additional latencies and stressing (meta data) servers from
>> thousands of clients. These lookup calls possibly can be avoided
>> in some cases. Incoming three patches address this issue.
>>
>>
>> Fist patch handles the case where we are creating a file with O_CREAT.
>> Before we go for file creation, we do a lookup on the file which is most
>> likely non-existent. After this lookup is done, we again go into libfuse
>> to create file. Such lookups where file is most likely non-existent, can
>> be avoided.
> 
> I'd really like to see a bit wider picture...
> 
> We have several cases, first of all let's look at plain O_CREAT
> without O_EXCL (assume that there were no changes since the last
> lookup for simplicity):
> 
> [not cached, negative]
>     ->atomic_open()
>        LOOKUP
>        CREATE
> 
> [not cached, positive]
>     ->atomic_open()
>        LOOKUP
>     ->open()
>        OPEN
> 
> [cached, negative, validity timeout not expired]
>     ->d_revalidate()
>        return 1
>     ->atomic_open()
>        CREATE
> 
> [cached, negative, validity timeout expired]
>     ->d_revalidate()
>        return 0
>     ->atomic_open()
>        LOOKUP
>        CREATE
> 
> [cached, positive, validity timeout not expired]
>     ->d_revalidate()
>        return 1
>     ->open()
>        OPEN
> 
> [cached, positive, validity timeout expired]
>     ->d_revalidate()
>        LOOKUP
>        return 1
>     ->open()
>        OPEN
> 
> (Caveat emptor: I'm just looking at the code and haven't actually
> tested what happens.)
> 
> Apparently in all of these cases we are doing at least one request, so
> it would make sense to make them uniform:
> 
> [not cached]
>     ->atomic_open()
>        CREATE_EXT
> 
> [cached]
>     ->d_revalidate()
>        return 0
>     ->atomic_open()
>        CREATE_EXT
> 
> Similarly we can look at the current O_CREAT | O_EXCL cases:
> 
> [not cached, negative]
>     ->atomic_open()
>        LOOKUP
>        CREATE
> 
> [not cached, positive]
>     ->atomic_open()
>        LOOKUP
>     return -EEXIST
> 
> [cached, negative]
>     ->d_revalidate()
>        return 0 (see LOOKUP_EXCL check)
>     ->atomic_open()
>        LOOKUP
>        CREATE
> 
> [cached, positive]
>     ->d_revalidate()
>        LOOKUP
>        return 1
>     return -EEXIST
> 
> Again we are doing at least one request, so we can unconditionally
> replace them with CREATE_EXT like the non-O_EXCL case.
> 
> 
>>
>> Second patch handles the case where we open first time a file/dir
>> but do a lookup first on it. After lookup is performed we make another
>> call into libfuse to open the file. Now these two separate calls into
>> libfuse can be combined and performed as a single call into libfuse.
> 
> And here's my analysis:
> 
> [not cached, negative]
>     ->lookup()
>        LOOKUP
>     return -ENOENT
> 
> [not cached, positive]
>     ->lookup()
>        LOOKUP
>     ->open()
>        OPEN
> 
> [cached, negative, validity timeout not expired]
>      ->d_revalidate()
>         return 1
>      return -ENOENT
> 
> [cached, negative, validity timeout expired]
>     ->d_revalidate()
>        return 0
>     ->atomic_open()
>        LOOKUP
>     return -ENOENT
> 
> [cached, positive, validity timeout not expired]
>     ->d_revalidate()
>        return 1
>     ->open()
>        OPEN
> 
> [cached, positive, validity timeout expired]
>     ->d_revalidate()
>        LOOKUP
>        return 1
>     ->open()
>        OPEN
> 
> There's one case were no request is sent:  a valid cached negative
> dentry.   Possibly we can also make this uniform, e.g.:
> 
> [not cached]
>     ->atomic_open()
>         OPEN_ATOMIC
> 
> [cached, negative, validity timeout not expired]
>      ->d_revalidate()
>         return 1
>      return -ENOENT
> 
> [cached, negative, validity timeout expired]
>     ->d_revalidate()
>        return 0
>     ->atomic_open()
>        OPEN_ATOMIC
> 
> [cached, positive]
>     ->d_revalidate()
>        return 0
>     ->atomic_open()
>        OPEN_ATOMIC
> 
> It may even make the code simpler to clearly separate the cases where
> the atomic variants are supported and when not.  I'd also consider
> merging CREATE_EXT into OPEN_ATOMIC, since a filesystem implementing
> one will highly likely want to implement the other as well.


Can you help me a bit to understand what we should change? I had also 
already thought to merge CREATE_EXT and OPEN_ATOMIC - so agreed.
Shall we make the other cases more visible?

Also thanks a lot for you revalidate patch.


Thanks,
Bernd

Thanks,
Bernd