Compressed files & the page cache

[Gao Xiang]

Hi Barry,

On 2025/7/21 09:02, Barry Song wrote:
> On Wed, Jul 16, 2025 at 8:28 AM Gao Xiang <hsiangkao at linux.alibaba.com> wrote:
>>

...

>>
>> ... high-order folios can cause side effects on embedded devices
>> like routers and IoT devices, which still have MiBs of memory (and I
>> believe this won't change due to their use cases) but they also use
>> Linux kernel for quite long time.  In short, I don't think enabling
>> large folios for those devices is very useful, let alone limiting
>> the minimum folio order for them (It would make the filesystem not
>> suitable any more for those users.  At least that is what I never
>> want to do).  And I believe this is different from the current LBS
>> support to match hardware characteristics or LBS atomic write
>> requirement.
> 
> Given the difficulty of allocating large folios, it's always a good
> idea to have order-0 as a fallback. While I agree with your point,
> I have a slightly different perspective — enabling large folios for
> those devices might be beneficial, but the maximum order should
> remain small. I'm referring to "small" large folios.

Yeah, agreed. Having a way to limit the maximum order for those small
devices (rather than disabling it completely) would be helpful.  At
least "small" large folios could still provide benefits when memory
pressure is light.

Thanks,
Gao Xiang

> 
> Still, even with those, allocation can be difficult — especially
> since so many other allocations (which aren't large folios) can cause
> fragmentation. So having order-0 as a fallback remains important.
> 
> It seems we're missing a mechanism to enable "small" large folios
> for files. For anon large folios, we do have sysfs knobs—though they
> don’t seem to be universally appreciated. :-)
> 
> Thanks
> Barry