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[2003:d8:2f23:5600:727b:b2f6:94a1:a51b]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3ee0fbc7410sm6681789f8f.35.2025.09.19.00.57.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 19 Sep 2025 00:57:27 -0700 (PDT) Message-ID: <7840f68e-7580-42cb-a7c8-1ba64fd6df69@redhat.com> Date: Fri, 19 Sep 2025 09:57:26 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v2 2/2] mm/khugepaged: abort collapse scan on guard PTEs To: Lance Yang Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, ioworker0@gmail.com, kirill@shutemov.name, hughd@google.com, mpenttil@redhat.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com References: <20250918050431.36855-1-lance.yang@linux.dev> <20250918050431.36855-3-lance.yang@linux.dev> From: David Hildenbrand Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: G4HSk-bf5gCMa1-cLdGyph4EWu_cSrWphLdR1DG0mjM_1758268649 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 087C1A0003 X-Stat-Signature: pwj6g4kg9tmgrycwz6z6jma9ocm5ihqr X-Rspam-User: X-HE-Tag: 1758268652-694199 X-HE-Meta: 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 jF+67P5Z 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 19.09.25 04:41, Lance Yang wrote: > > > On 2025/9/19 02:47, David Hildenbrand wrote: >> On 18.09.25 07:04, Lance Yang wrote: >>> From: Lance Yang >>> >>> Guard PTE markers are installed via MADV_GUARD_INSTALL to create >>> lightweight guard regions. >>> >>> Currently, any collapse path (khugepaged or MADV_COLLAPSE) will fail when >>> encountering such a range. >>> >>> MADV_COLLAPSE fails deep inside the collapse logic when trying to swap-in >>> the special marker in __collapse_huge_page_swapin(). >>> >>> hpage_collapse_scan_pmd() >>>   `- collapse_huge_page() >>>       `- __collapse_huge_page_swapin() -> fails! >>> >>> khugepaged's behavior is slightly different due to its max_ptes_swap >>> limit >>> (default 64). It won't fail as deep, but it will still needlessly scan up >>> to 64 swap entries before bailing out. >>> >>> IMHO, we can and should detect this much earlier. >>> >>> This patch adds a check directly inside the PTE scan loop. If a guard >>> marker is found, the scan is aborted immediately with >>> SCAN_PTE_NON_PRESENT, >>> avoiding wasted work. >>> >>> Suggested-by: Lorenzo Stoakes >>> Signed-off-by: Lance Yang >>> --- >>>   mm/khugepaged.c | 10 ++++++++++ >>>   1 file changed, 10 insertions(+) >>> >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>> index 9ed1af2b5c38..70ebfc7c1f3e 100644 >>> --- a/mm/khugepaged.c >>> +++ b/mm/khugepaged.c >>> @@ -1306,6 +1306,16 @@ static int hpage_collapse_scan_pmd(struct >>> mm_struct *mm, >>>                       result = SCAN_PTE_UFFD_WP; >>>                       goto out_unmap; >>>                   } >>> +                /* >>> +                 * Guard PTE markers are installed by >>> +                 * MADV_GUARD_INSTALL. Any collapse path must >>> +                 * not touch them, so abort the scan immediately >>> +                 * if one is found. >>> +                 */ >>> +                if (is_guard_pte_marker(pteval)) { >>> +                    result = SCAN_PTE_NON_PRESENT; >>> +                    goto out_unmap; >>> +                } >> >> Thinking about it, this is interesting. >> >> Essentially we track any non-swap swap entries towards >> khugepaged_max_ptes_swap, which is rather weird. >> >> I think we might also run into migration entries here and hwpoison entries? >> >> So what about just generalizing this: >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index af5f5c80fe4ed..28f1f4bf0e0a8 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1293,7 +1293,24 @@ static int hpage_collapse_scan_pmd(struct >> mm_struct *mm, >>         for (_address = address, _pte = pte; _pte < pte + HPAGE_PMD_NR; >>              _pte++, _address += PAGE_SIZE) { >>                 pte_t pteval = ptep_get(_pte); >> -               if (is_swap_pte(pteval)) { >> + >> +               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >> +                       ++none_or_zero; >> +                       if (!userfaultfd_armed(vma) && >> +                           (!cc->is_khugepaged || >> +                            none_or_zero <= khugepaged_max_ptes_none)) { >> +                               continue; >> +                       } else { >> +                               result = SCAN_EXCEED_NONE_PTE; >> +                               count_vm_event(THP_SCAN_EXCEED_NONE_PTE); >> +                               goto out_unmap; >> +                       } >> +               } else if (!pte_present(pteval)) { >> +                       if (non_swap_entry(pte_to_swp_entry(pteval))) { >> +                               result = SCAN_PTE_NON_PRESENT; >> +                               goto out_unmap; >> +                       } >> + >>                         ++unmapped; >>                         if (!cc->is_khugepaged || >>                             unmapped <= khugepaged_max_ptes_swap) { >> @@ -1313,18 +1330,7 @@ static int hpage_collapse_scan_pmd(struct >> mm_struct *mm, >>                                 goto out_unmap; >>                         } >>                 } >> -               if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) { >> -                       ++none_or_zero; >> -                       if (!userfaultfd_armed(vma) && >> -                           (!cc->is_khugepaged || >> -                            none_or_zero <= khugepaged_max_ptes_none)) { >> -                               continue; >> -                       } else { >> -                               result = SCAN_EXCEED_NONE_PTE; >> -                               count_vm_event(THP_SCAN_EXCEED_NONE_PTE); >> -                               goto out_unmap; >> -                       } >> -               } >> + >>                 if (pte_uffd_wp(pteval)) { >>                         /* >>                          * Don't collapse the page if any of the small >> >> >> With that, the function flow looks more similar to >> __collapse_huge_page_isolate(), >> except that we handle swap entries in there now. > > Ah, indeed. I like this crazy idea ;p > >> >> >> And with that in place, couldn't we factor out a huge chunk of both >> scanning >> functions into some helper (passing whether swap entries are allowed or >> not?). > > Yes. Factoring out the common scanning logic into a new helper is a > good suggestion. It would clean things up ;) > >> >> Yes, I know, refactoring khugepaged, crazy idea. > > I'll look into that. But let's do this separately :) Right, but let's just skip any non-swap entries early in this patch instead of special-casing only guard ptes. -- Cheers David / dhildenb