Re: [PATCH v14 00/12] FUSE passthrough for file io

From: Antonio SJ Musumeci <trapexit@spawn.link>
To: Bernd Schubert <bernd.schubert@fastmail.fm>,
	Amir Goldstein <amir73il@gmail.com>,
	Miklos Szeredi <miklos@szeredi.hu>
Cc: Daniel Rosenberg <drosen@google.com>,
	Paul Lawrence <paullawrence@google.com>,
	Alessio Balsini <balsini@android.com>,
	Christian Brauner <brauner@kernel.org>,
	fuse-devel@lists.sourceforge.net, linux-fsdevel@vger.kernel.org,
	Dave Chinner <david@fromorbit.com>
Subject: Re: [PATCH v14 00/12] FUSE passthrough for file io
Date: Wed, 29 Nov 2023 21:39:21 +0000	[thread overview]
Message-ID: <44ff6b37-7c4b-485d-8ebf-de5fadd3c527@spawn.link> (raw)
In-Reply-To: <2f6513fa-68d8-43c8-87a4-62416c3e1bfd@fastmail.fm>

On 11/29/23 14:46, Bernd Schubert wrote:
> 
> 
> On 11/29/23 18:39, Amir Goldstein wrote:
>> On Wed, Nov 29, 2023 at 6:55 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>>>
>>> On Wed, 29 Nov 2023 at 16:52, Amir Goldstein <amir73il@gmail.com> wrote:
>>>
>>>> direct I/O read()/write() is never a problem.
>>>>
>>>> The question is whether mmap() on a file opened with FOPEN_DIRECT_IO
>>>> when the inode is in passthrough mode, also uses fuse_passthrough_mmap()?
>>>
>>> I think it should.
>>>
>>>> or denied, similar to how mmap with ff->open_flags & FOPEN_DIRECT_IO &&
>>>> vma->vm_flags & VM_MAYSHARE) && !fc->direct_io_relax
>>>> is denied?
>>>
>>> What would be the use case for FOPEN_DIRECT_IO with passthrough mmap?
>>>
>>
>> I don't have a use case. That's why I was wondering if we should
>> support it at all, but will try to make it work.
> 
> What is actually the use case for FOPEN_DIRECT_IO and passthrough?
> Avoiding double page cache?
> 
>>
>>>> A bit more challenging, because we will need to track unmounts, or at
>>>> least track
>>>> "was_cached_mmaped" state per file, but doable.
>>>
>>> Tracking unmaps via fuse_vma_close() should not be difficult.
>>>
>>
>> OK. so any existing mmap, whether on FOPEN_DIRECT_IO or not
>> always prevents an inode from being "neutral".
>>
> 
> 
> Thanks,
> Bernd
> 

 > Avoiding double page cache?

Currently my users enable direct_io because 1) it is typically 
materially faster than not 2) avoiding double page caching (it is a 
union filesystem).

The only real reason people disable direct_io is because many apps need 
shared mmap. I've implemented a mode to lookup details about the 
requesting app and optionally disable direct_io for apps which are known 
to need shared mmap but that isn't ideal. The relaxed mode being 
discussed would likely be more performant and more transparent to the 
user. That transparency is nice if that can continue as it is already 
pretty difficult to explain all these options to the layman.

Offtopic: What happens in passthrough mode when an error occurs? 
Currently I have a number of behaviors relying on the fact that I can 
intercept and respond to errors. I think my users will gladly give them 
up for near native io perf but if they can get both it would be nice.