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b=fnT5B7rZs+/H+B2XxDgLdwpnqBsdgLJlW6caGO6+uMkQcnOB7sEv4sGYteVV2ZS0j TNZYFD4PxI+3UfK2iUx+3OO8wICEXJvBLcO13EioFdbCr6OL93dyPE7+NYf9XJamU8 VqYo4chwV4Ih4OM5rR6+FrlC1O5u6y+mHV79Rwo7ET8RNSBTG/c15Xbya4S+Axj3VN jUl9GoHg8MzZWHSVnoZRdfv/14gGEDs//AvyyqGL5T7RnVc+K/Qdk0FS0Du8uBfLIm MWmhAkFP+NmSzgHj134/5Sa+1R836FR1DFmjtROiQdB7nIr0PnXQf8E8M2A7P4TESJ IN0A2a9fPYQuw== Message-ID: <0a860f74-b2fc-4ea7-9f13-1879c2c8c168@kernel.org> Date: Mon, 11 May 2026 10:01:39 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/khugepaged: avoid underflow in madvise_collapse for sub-PMD MADV_COLLAPSE To: Wandun , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev References: <20260511065701.799006-1-chenwandun@lixiang.com> <4e3d0c1b-af33-470d-acb0-1e6540ba312a@gmail.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Queue-Id: 5A7FB14000F X-Rspamd-Server: rspam04 X-Stat-Signature: 57xsrekfojfy9e4qrcf55nquchr7jf4f X-HE-Tag: 1778486504-760878 X-HE-Meta: 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 p3MpA+R5 Qgk6nPB7uLuG9TtId8ehOJhIqvebOwCSDIonVckNtoYU6qSfnfss+uuwKnrEkfMgIFNg2XdrbqVQAuBWb/dn87nRtCvM67n8GlwnxiKTEytfc6BDJhAt38L6eRmIQRu1p7EszA2tYwyr8DhX1iWf1fmaXsaL93dVlIybbKHHQGaGCQJcJbz7kZQwoXUgtEHN3uOqoUKof2jqIptUJvHWtd0woowoxb73ub/lL3yjTYQduaDvDDmPIBWOeQuFklwGqU85OKzgGaBRolFdjBJESOkE0wXtPNT4JB1xy3I7Dmk2THGXg6OMaKHs17ZroWfZ4qbPFw2+l9LgfHOHQ8sNBKOG7cy6SQr0PdlbZuJOMyJHgcYuSyrkawvI4y1Bo7eSP/zOXDpBXrj7byaI= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/11/26 09:35, Wandun wrote: > > > On 5/11/26 15:17, David Hildenbrand (Arm) wrote: >> On 5/11/26 08:57, Wandun Chen wrote: >>> From: Chen Wandun >>> >>> madvise_collapse() computes the THP-aligned window: >>> >>>      hstart = ALIGN(start, HPAGE_PMD_SIZE);     /* round up */ >>>      hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE);    /* round down */ >>> >>> The following case will cause hstart > hend, and result in underflow >>> in the return statement, avoid it by returning -EINVAL early when >>> hstart > hend. >>> >>>      madvise(PMD-aligned + PAGE_SIZE, PAGE_SIZE, MADV_COLLAPSE); >> Ok, so providing a PMD-aligned address as start will result in 0 and a >> non-aligned address will result in -EINVAL. >> >> Didn't Lorenzo agree that just returning 0 in both cases would be clearer? But I >> might have misunderstood it. > Lorenzo suggested retuern -EINVAL for both case at the beginning, > Later, Lorenzo add an correction, suggested should return 0 for > compatibilty reasons for hstart == hend case. > (If I haven't missed any information) Let's wait for Lorenzo's confirmation. I think the important part is that we cannot have a situation where start < end (given that madvise() consumes a length). Because, there we really should have returned -EINVAL. For start <= end, if there is nothing suitable to collapse, I'd say we'd just consistently return 0. -- Cheers, David