On 10 Jun 2025, at 10:03, Lorenzo Stoakes wrote: > On Mon, Jun 09, 2025 at 03:49:52PM -0400, Zi Yan wrote: > [snip] >>> I really think a hard cap, expressed in KB/MB, on pageblock size is the way to >>> go (but overrideable for people crazy enough to truly want 512 MB pages - and >>> who cannot then complain about watermarks). >> >> I agree. Basically, I am thinking: >> 1) use something like 2MB as default pageblock size for all arch (the value can >> be set differently if some arch wants a different pageblock size due to other reasons), this can be done by modifying PAGE_BLOCK_MAX_ORDER’s default >> value; > > I don't think we can set this using CONFIG_PAGE_BLOCK_MAX_ORDER. > > Because the 'order' will be a different size depending on page size obviously. > > So I'm not sure how this would achieve what we want? > > It seems to me we should have CONFIG_PAGE_BLOCK_MAX_SIZE_MB or something like > this, and we take min(page_size << CONFIG_PAGE_BLOCK_MAX_ORDER, > CONFIG_PAGE_BLOCK_MAX_SIZE_MB << 20) as the size. OK. Now I get what you mean. Yeah, using MB is clearer as user does not need to know page size to set the right pageblock size. > >> >> 2) make pageblock_order a boot time parameter, so that user who wants >> 512MB pages can still get it by changing pageblock order at boot time. >> > > Again, I don't think order is the right choice here, though having it boot time > configurable (perhaps overriding the default config there) seems sensible. Understood. The new pageblock size should be set using MB. > >> WDYT? > >> >>> >>>> >>>> Often, user just ask for an impossible combination: they >>>> want to use all free memory, because they paid for it, and they >>>> want THPs, because they want max performance. When PMD THP is >>>> small like 2MB, the “unusable” free memory is not that noticeable, >>>> but when PMD THP is as large as 512MB, user just cannot unsee it. :) >>> >>> Well, users asking for crazy things then being surprised when they get them >>> is nothing new :P >>> >>>> >>>> >>>> Best Regards, >>>> Yan, Zi >>> >>> Thanks for your input! >>> >>> Cheers, Lorenzo >> >> >> Best Regards, >> Yan, Zi Best Regards, Yan, Zi
