linux-kernel - Re: [PATCH RESEND V11 0/7] fuse: Add support for passthrough read/write

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Date:   Mon, 25 Jan 2021 10:39:52 +0800
From:   "wu-yan@....com" <wu-yan@....com>
To:     Alessio Balsini <balsini@...roid.com>
CC:     <akailash@...gle.com>, <amir73il@...il.com>, <axboe@...nel.dk>,
        <bergwolf@...il.com>, <duostefano93@...il.com>,
        <dvander@...gle.com>, <fuse-devel@...ts.sourceforge.net>,
        <gscrivan@...hat.com>, <jannh@...gle.com>,
        <kernel-team@...roid.com>, <linux-fsdevel@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <maco@...roid.com>,
        <miklos@...redi.hu>, <palmer@...belt.com>,
        <paullawrence@...gle.com>, <trapexit@...wn.link>,
        <zezeozue@...gle.com>
Subject: Re: [PATCH RESEND V11 0/7] fuse: Add support for passthrough
 read/write

On 1/22/21 7:06 PM, Alessio Balsini wrote:
> On Tue, Jan 19, 2021 at 12:34:23PM +0000, Alessio Balsini wrote:
>> On Tue, Jan 19, 2021 at 07:06:54PM +0800, Rokudo Yan wrote:
>>> on Mon, Jan 18, 2021 at 5:27 PM Alessio Balsini <balsini@...roid.com> wrote:
>>>>
>>>> This is the 11th version of the series, rebased on top of v5.11-rc4.
>>>> Please find the changelog at the bottom of this cover letter.
>>>>
>>>> Add support for file system passthrough read/write of files when enabled
>>>> in userspace through the option FUSE_PASSTHROUGH.
>>> [...]
>>>
>>>
>>> Hi Allesio,
>>>
>>> Could you please add support for passthrough mmap too ?
>>> If the fuse file opened with passthrough actived, and then map (shared) to (another) process
>>> address space using mmap interface. As access the file with mmap will pass the vfs cache of fuse,
>>> but access the file with read/write will bypass the vfs cache of fuse, this may cause inconsistency.
>>> eg. the reader read the fuse file with mmap() and the writer modify the file with write(), the reader
>>> may not see the modification immediately since the writer bypass the vfs cache of fuse.
>>> Actually we have already meet an issue caused by the inconsistency after applying fuse passthrough
>>> scheme to our product.
>>>
>>> Thanks,
>>> yanwu.
>>
>> Hi yanwu,
>>
>> Thank you for your interest in this change.
>>
>> FUSE passthrough for mmap is an extension that is already in my TODO
>> list, together with passthrough for directories.
>> For now I would prefer to keep this series minimal to make the review
>> process leaner and simpler.
>> I will start working on extending this series with new features and
>> addressing more corner cases as soon as these changes get merged, what
>> do you think?
>>
>> Thanks,
>> Alessio
> 
> Hi yanwu,
> 
> Sorry if I overlooked this issue. I added memory-mapping to my tests and
> could reproduce/verify this wrong behavior you mentioned.
> 
> I created this WIP (history may change) branch that has the missing mmap
> implementation:
> 
>    https://github.com/balsini/linux/commits/fuse-passthrough-v12-develop-v5.11-rc4
> 
> I did some mmap testing in the last days with this extra mmap
> implementation and couldn't find any issues, everything seems to be
> working as expected with the extra mmap patch. Can you please confirm
> this is fixed on your end too?
> I'm also going to revert in this branch the stacking policy changes to
> how they were in V10 as suggested by Amir if there are no concerns with
> that.
> I'm waiting for some extra tests to complete and, if no issue is
> detected, I'll post the V12 series super soon.
> 
> Thanks,
> Alessio
> 

Hi, Alessio

Thank you for your reply. I have already added mmap for passthrough in 
our product, and the issue mentioned before is fixed. And I will update 
if any new issuses found.

Thanks
yanwu