Re: [PATCH v2 RESEND] fuse: Enable dynamic configuration of fuse max pages limit (FUSE_MAX_MAX_PAGES)

[Jingbo Xu <jefflexu@linux.alibaba.com>]

On 9/6/24 5:16 AM, Bernd Schubert wrote:
> Hi Joanne,
> 
> On 9/5/24 19:45, Joanne Koong wrote:
>> Introduce the capability to dynamically configure the fuse max pages
>> limit (formerly #defined as FUSE_MAX_MAX_PAGES) through a sysctl.
>> This enhancement allows system administrators to adjust the value
>> based on system-specific requirements.
>>
>> This removes the previous static limit of 256 max pages, which limits
>> the max write size of a request to 1 MiB (on 4096 pagesize systems).
>> Having the ability to up the max write size beyond 1 MiB allows for the
>> perf improvements detailed in this thread [1].
> 
> the change itself looks good to me, but have you seen this discussion here?
> 
> https://lore.kernel.org/lkml/CAJfpegs10SdtzNXJfj3=vxoAZMhksT5A1u5W5L6nKL-P2UOuLQ@mail.gmail.com/T/
> 
> 
> Miklos is basically worried about page pinning and accounting for that
> for unprivileged user processes. 

Yeah this is the blocking issue here when attempting to increase the max
pages limit.

For background requests, maybe this could be fixed by taking the max
pages limit into account when calculating max_user_bgreq. (currently
max_user_bgreq is calculated "assuming 392 bytes per request")

While for synchronous requests, I'm not sure if this indeed matters.  Or
at least it won't be such severe as the pinned memory is limited by the
number of the user process in this case.

-- 
Thanks,
Jingbo