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usamaarif642@gmail.com, sunnanyong@huawei.com, vishal.moola@gmail.com, thomas.hellstrom@linux.intel.com, yang@os.amperecomputing.com, kas@kernel.org, aarcange@redhat.com, raquini@redhat.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, tiwai@suse.de, will@kernel.org, dave.hansen@linux.intel.com, jack@suse.cz, cl@gentwo.org, jglisse@google.com, surenb@google.com, zokeefe@google.com, hannes@cmpxchg.org, rientjes@google.com, mhocko@suse.com, rdunlap@infradead.org, hughd@google.com, richard.weiyang@gmail.com, lance.yang@linux.dev, vbabka@suse.cz, rppt@kernel.org, jannh@google.com, pfalcato@suse.de Subject: Re: [PATCH v12 mm-new 06/15] khugepaged: introduce collapse_max_ptes_none helper function Message-ID: <2ab547d2-584b-48fe-9f43-7c14caa7ab05@lucifer.local> References: <20251022183717.70829-1-npache@redhat.com> <20251022183717.70829-7-npache@redhat.com> <5f8c69c1-d07b-4957-b671-b37fccf729f1@lucifer.local> <74583699-bd9e-496c-904c-ce6a8e1b42d9@redhat.com> 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Wang wrote: > I finally finished reading through the discussions across multiple > threads:), and it looks like we've reached a preliminary consensus (make > 0/511 work). Great and thanks! Yes we're getting there :) it's a sincere effort to try to find a way to move forwards. > > IIUC, the strategy is, configuring it to 511 means always enabling mTHP > collapse, configuring it to 0 means collapsing mTHP only if all PTEs are > non-none/zero, and for other values, we issue a warning and prohibit mTHP > collapse (avoid Lorenzo's concern about silently changing max_ptes_none). > Then the implementation for collapse_max_ptes_none() should be as follows: > > static int collapse_max_ptes_none(unsigned int order, bool full_scan) > { > /* ignore max_ptes_none limits */ > if (full_scan) > return HPAGE_PMD_NR - 1; > > if (order == HPAGE_PMD_ORDER) > return khugepaged_max_ptes_none; > > /* > * To prevent creeping towards larger order collapses for mTHP > collapse, > * we restrict khugepaged_max_ptes_none to only 511 or 0, > simplifying the > * logic. This means: > * max_ptes_none == 511 -> collapse mTHP always > * max_ptes_none == 0 -> collapse mTHP only if we all PTEs are > non-none/zero > */ > if (!khugepaged_max_ptes_none || khugepaged_max_ptes_none == > HPAGE_PMD_NR - 1) > return khugepaged_max_ptes_none >> (HPAGE_PMD_ORDER - > order); > > pr_warn_once("mTHP collapse only supports khugepaged_max_ptes_none > configured as 0 or %d\n", HPAGE_PMD_NR - 1); > return -EINVAL; > } > > So what do you think? Yeah I think something like this. Though I'd implement it more explicitly like: /* Zero/non-present collapse disabled. */ if (!khugepaged_max_ptes_none) return 0; /* Collapse the maximum number of zero/non-present PTEs. */ if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) return (1 << order) - 1; Then we can do away with this confusing (HPAGE_PMD_ORDER - order) stuff. A quick check in google sheets suggests my maths is ok here but do correct me if I'm wrong :) Cheers, Lorenzo