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.
