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

[Bernd Schubert]

On 5/19/22 11:39, Miklos Szeredi wrote:
> On Tue, 17 May 2022 at 12:08, Dharmendra Singh <dharamhans87@xxxxxxxxx> 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