Re: [PATCH 0/5] add STATIC_PMD_ZERO_PAGE config option

[Dave Hansen <dave.hansen@intel.com>]

On 6/12/25 13:36, Pankaj Raghav (Samsung) wrote:
> On Thu, Jun 12, 2025 at 06:50:07AM -0700, Dave Hansen wrote:
>> On 6/12/25 03:50, Pankaj Raghav wrote:
>>> But to use huge_zero_folio, we need to pass a mm struct and the
>>> put_folio needs to be called in the destructor. This makes sense for
>>> systems that have memory constraints but for bigger servers, it does not
>>> matter if the PMD size is reasonable (like in x86).
>>
>> So, what's the problem with calling a destructor?
>>
>> In your last patch, surely bio_add_folio() can put the page/folio when
>> it's done. Is the real problem that you don't want to call zero page
>> specific code at bio teardown?
> 
> Yeah, it feels like a lot of code on the caller just to use a zero page.
> It would be nice just to have a call similar to ZERO_PAGE() in these
> subsystems where we can have guarantee of getting huge zero page.
> 
> Apart from that, these are the following problems if we use
> mm_get_huge_zero_folio() at the moment:
> 
> - We might end up allocating 512MB PMD on ARM systems with 64k base page
>   size, which is undesirable. With the patch series posted, we will only
>   enable the static huge page for sane architectures and page sizes.

Does *anybody* want the 512MB huge zero page? Maybe it should be an
opt-in at runtime or something.

> - In the current implementation we always call mm_put_huge_zero_folio()
>   in __mmput()[1]. I am not sure if model will work for all subsystems. For
>   example bio completions can be async, i.e, we might need a reference
>   to the zero page even if the process is no longer alive.

The mm is a nice convenient place to stick an mm but there are other
ways to keep an efficient refcount around. For instance, you could just
bump a per-cpu refcount and then have the shrinker sum up all the
refcounts to see if there are any outstanding on the system as a whole.

I understand that the current refcounts are tied to an mm, but you could
either replace the mm-specific ones or add something in parallel for
when there's no mm.