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b=vDmvFmZ+iTrW6rG0Gcm9nWDPvvRK1Rsj48dFtKprQeQ1e6lYyM5VMFRhBpg8rwS61 0TNXTtczIM+BVYe4dk6R82ZVaWwaFbuN+I3al6alVhDH1MljGD3eYqKFw/QXi7Qvm0 JKoE6kG5Db51dOjr9Juw7dqsEFy3nED8MYgsGtG2paTjp5kcRM6tNacnnxJWHErq5S M8Qrai0JHOMsBE7ZTTpOx6tIkJOYjnAKZFKHD4doWsSaZi9ZYNp4SbjR5D1f4rsOHj H3beDvWRIUdalnXZNIQ+HM/9w5MnR/fIEerEMaPESJ8Zc3zgsQGlP1eDVwfo09KpCR ofuvouMs7WwgQ== Date: Mon, 18 May 2026 20:32:50 +0100 From: Lorenzo Stoakes To: "David Hildenbrand (Arm)" Cc: Wei Yang , Lance Yang , npache@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, liam@infradead.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com Subject: Re: [PATCH mm-unstable v17 04/14] mm/khugepaged: generalize __collapse_huge_page_* for mTHP support Message-ID: References: <20260511185817.686831-5-npache@redhat.com> <20260512074202.10253-1-lance.yang@linux.dev> <20260514031009.f66cgop3ctgiqxz3@master> <9b33339e-157a-45b7-942e-3be3418a5142@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9b33339e-157a-45b7-942e-3be3418a5142@kernel.org> X-Rspam-User: X-Rspamd-Queue-Id: CFECE180004 X-Rspamd-Server: rspam04 X-Stat-Signature: aypdhz6xd39nq1f9qpeintu4fizrkcch X-HE-Tag: 1779132786-493896 X-HE-Meta: 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 2ww5DnqX 2mT4fVt5k5yQR7EqubN43C5lhzTvAYnMUv5P0ulgaWUyJ015Svv+/JDtb7mJR/od8NSg8jqgal2yKCJLfuo7I2HTHd6yIeDqx9uVBoSYzwkDXjIdCaziWc9pQw5cAR26rl3uBpI3/zL3W/uPc7Xn8eiJdk/2bR1e5UvvDmvqMImhEyttViX9s8rL66cwawu2VE7drFvClpF/d1qu+P2bZNyvH8+ou19PtXNpx9WaOwO7bIRc1mma2KPEojQ3yh/AwvJWdqgwtajega0npII7uCCllrvMbwuL5jgeeN4s9WTtWNkmCiPfLIx8rgoLTPKEL8WA+yFVSWIxXkuKKwXpzXw6NF1Mj1jHfWp/tmNwEPtoF9KbhEQti42YmYRK9N+cxQEmeWY0XtfMbQFAzWZIuaVgL80Hln3OXQYzITc470V7vR+kddSF8H0X6L/i2xf390p4N6ORb443qVci+MRJuW97V3LiEf/NhOtVY Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, May 18, 2026 at 03:16:11PM +0200, David Hildenbrand (Arm) wrote: > On 5/14/26 05:10, Wei Yang wrote: > > On Tue, May 12, 2026 at 03:42:02PM +0800, Lance Yang wrote: > >> > >> On Mon, May 11, 2026 at 12:58:04PM -0600, Nico Pache wrote: > >>> generalize the order of the __collapse_huge_page_* and collapse_max_* > >>> functions to support future mTHP collapse. > >>> > >>> The current mechanism for determining collapse with the > >>> khugepaged_max_ptes_none value is not designed with mTHP in mind. This > >>> raises a key design issue: if we support user defined max_pte_none values > >>> (even those scaled by order), a collapse of a lower order can introduces > >>> an feedback loop, or "creep", when max_ptes_none is set to a value greater > >>> than HPAGE_PMD_NR / 2. [1] > >>> > >>> With this configuration, a successful collapse to order N will populate > >>> enough pages to satisfy the collapse condition on order N+1 on the next > >>> scan. This leads to unnecessary work and memory churn. > >>> > >>> To fix this issue introduce a helper function that will limit mTHP > >>> collapse support to two max_ptes_none values, 0 and HPAGE_PMD_NR - 1. > >>> This effectively supports two modes: [2] > >>> > >>> - max_ptes_none=0: never collapses if it encounters an empty PTE or a PTE > >>> that maps the shared zeropage. Consequently, no memory bloat. > >>> - max_ptes_none=511 (on 4k pagesz): Always collapse to the highest > >>> available mTHP order. > >>> > >>> This removes the possiblilty of "creep", while not modifying any uAPI > >>> expectations. A warning will be emitted if any non-supported > >>> max_ptes_none value is configured with mTHP enabled. > >>> > >>> mTHP collapse will not honor the khugepaged_max_ptes_shared or > >>> khugepaged_max_ptes_swap parameters, and will fail if it encounters a > >>> shared or swapped entry. > >>> > >>> No functional changes in this patch; however it defines future behavior > >>> for mTHP collapse. > >>> > >>> [1] - https://lore.kernel.org/all/e46ab3ab-a3d7-4fb7-9970-d0704bd5d05a@arm.com > >>> [2] - https://lore.kernel.org/all/37375ace-5601-4d6c-9dac-d1c8268698e9@redhat.com > >>> > >>> Co-developed-by: Dev Jain > >>> Signed-off-by: Dev Jain > >>> Signed-off-by: Nico Pache > >>> --- > >>> include/trace/events/huge_memory.h | 3 +- > >>> mm/khugepaged.c | 117 ++++++++++++++++++++--------- > >>> 2 files changed, 85 insertions(+), 35 deletions(-) > >>> > >>> diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h > >>> index bcdc57eea270..443e0bd13fdb 100644 > >>> --- a/include/trace/events/huge_memory.h > >>> +++ b/include/trace/events/huge_memory.h > >>> @@ -39,7 +39,8 @@ > >>> EM( SCAN_STORE_FAILED, "store_failed") \ > >>> EM( SCAN_COPY_MC, "copy_poisoned_page") \ > >>> EM( SCAN_PAGE_FILLED, "page_filled") \ > >>> - EMe(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback") > >>> + EM(SCAN_PAGE_DIRTY_OR_WRITEBACK, "page_dirty_or_writeback") \ > >>> + EMe(SCAN_INVALID_PTES_NONE, "invalid_ptes_none") > >>> > >>> #undef EM > >>> #undef EMe > >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >>> index f68853b3caa7..27465161fa6d 100644 > >>> --- a/mm/khugepaged.c > >>> +++ b/mm/khugepaged.c > >>> @@ -61,6 +61,7 @@ enum scan_result { > >>> SCAN_COPY_MC, > >>> SCAN_PAGE_FILLED, > >>> SCAN_PAGE_DIRTY_OR_WRITEBACK, > >>> + SCAN_INVALID_PTES_NONE, > >>> }; > >>> > >>> #define CREATE_TRACE_POINTS > >>> @@ -353,37 +354,60 @@ static bool pte_none_or_zero(pte_t pte) > >>> * PTEs for the given collapse operation. > >>> * @cc: The collapse control struct > >>> * @vma: The vma to check for userfaultfd > >>> + * @order: The folio order being collapsed to > >>> * > >>> * Return: Maximum number of none-page or zero-page PTEs allowed for the > >>> * collapse operation. > >>> */ > >>> -static unsigned int collapse_max_ptes_none(struct collapse_control *cc, > >>> - struct vm_area_struct *vma) > >>> +static int collapse_max_ptes_none(struct collapse_control *cc, > >>> + struct vm_area_struct *vma, unsigned int order) > >>> { > >>> + unsigned int max_ptes_none = khugepaged_max_ptes_none; > >>> // If the vma is userfaultfd-armed, allow no none-page or zero-page PTEs. > >> > >> One thing I still want to call out: kernel code usually uses C-style > >> comments :) > >> > >>> if (vma && userfaultfd_armed(vma)) > >>> return 0; > >>> // for MADV_COLLAPSE, allow any none-page or zero-page PTEs. > >>> if (!cc->is_khugepaged) > >>> return HPAGE_PMD_NR; > >>> - // For all other cases repect the user defined maximum. > >>> - return khugepaged_max_ptes_none; > >>> + // for PMD collapse, respect the user defined maximum. > >>> + if (is_pmd_order(order)) > >>> + return max_ptes_none; > >>> + /* Zero/non-present collapse disabled. */ > >>> + if (!max_ptes_none) > >>> + return 0; > >>> + // for mTHP collapse with the sysctl value set to KHUGEPAGED_MAX_PTES_LIMIT, > >>> + // scale the maximum number of PTEs to the order of the collapse. > >>> + if (max_ptes_none == KHUGEPAGED_MAX_PTES_LIMIT) > >>> + return (1 << order) - 1; > >>> + > >>> + // We currently only support max_ptes_none values of 0 or KHUGEPAGED_MAX_PTES_LIMIT. > >>> + // Emit a warning and return -EINVAL. > >>> + pr_warn_once("mTHP collapse only supports max_ptes_none values of 0 or %u\n", > >>> + KHUGEPAGED_MAX_PTES_LIMIT); > >> > >> Maybe fallback to 0 instead, as David suggested earlier? > >> > > > > It looks reasonable to fallback to 0. > > > > But as the updated Document says in patch 14: > > > > For mTHP collapse, only 0 or (HPAGE_PMD_NR - 1) are supported. Any other > > value will emit a warning and no mTHP collapse will be attempted. > > > > This is why it does like this now. > > > > mthp_collapse() > > max_ptes_none = collapse_max_ptes_none(); > > if (max_ptes_none < 0) > > return collapsed; > > > >> max_ptes_none is mostly legacy PMD THP behavior. mTHP is new, and any > >> intermediate value in (0, KHUGEPAGED_MAX_PTES_LIMIT) would implicitly > >> disable it :( > >> > > > > So it depends on what we want to do here :-) > > > > For me, I would vote for fallback to 0. > > At this point I'll prefer to not return errors from collapse_max_ptes_none(). > It's just rather awkward to return an error deep down in collapse code for a > configuration problem. > > For mthp collapse, we only support max_ptes_none==0 and > max_ptes_none=="HPAGE_PMD_NR - 1" (default). > > If another value is specified while collapsing mTHP, print a warning and treat > it as 0 (save value, no creep, no memory waste). > > In a sense, this is similar to how we handle max_ptes_shared + max_ptes_swap: > for mTHP: we always treat them as being 0 for mTHP collapse (and don't issue a > warning, because we would issue a warning with the default settings). > > @Lorenzo, fine with you? Yes 100%, this sounds sensible both in terms of the error and the default. Let's keep our lives simple(-ish) please :) > > -- > Cheers, > > David Cheers, Lorenzo