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Sat, 29 Aug 2026 04:15:32 +0000 X-Mizu-Trace-ID: b8a438c168cc2223 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: kas@kernel.org, jthoughton@google.com Cc: lance.yang@linux.dev, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, shy828301@gmail.com, zokeefe@google.com, hughd@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/khugepaged: Don't collapse uffd-minor-registered VMAs Date: Sat, 29 Aug 2026 12:15:23 +0800 Message-Id: <20260829041523.43008-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: BBDA71A0006 X-Stat-Signature: 9daowg35j69n9nuw97ng3idadtciz3as X-Rspam-User: X-HE-Tag: 1787976945-943352 X-HE-Meta: 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 2Klwojh5 4ZfO0QEx0pX1oiSLTBhkP8tEhW+d7Zz+Wh/gQX9qdYBT+dfIgCrdNVPY7rX+UCrIlCxoX0lfOXC0CExB2eRzqZO7PChr/YwS+OtagYGN2b4686WpXELiHU6j25POPJU/o0CT25t/afRJ6qz2CchVmIRaJsI5XTeEP/1QuzK6AaQhy7S8iHcDlxA+/8LJa8u9JEFl+PwG7o6v4FF0N9Xt7HDZJeISfwIi+0xPBiCkkK+xHCLo= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Aug 28, 2026 at 02:07:07PM +0100, Kiryl Shutsemau wrote: >On Fri, Aug 28, 2026 at 05:47:03PM +0800, Lance Yang wrote: >> >> On Fri, Aug 28, 2026 at 12:50:04AM +0000, James Houghton wrote: >> >Userfaultfd minor faults provides userspace with the ability to manually >> >install PTEs with UFFDIO_CONTINUE. Right now, khugepaged collapse can >> >map holes in the VMA when a naturally-aligned THP is present without >> >explicit action from userspace. >> > >> >This is a problem, as it bypasses userfaultfd minor faults that >> >userspace is expecting to handle. >> >> One basic question first. Should MADV_COLLAPSE refuse to collapse a >> UFFD-minor-registered VMA, regardless of whether all PTEs are present? > >I think the line is not between MADV_COLLAPSE vs khugepaged, it is if >vma->vm_mm is current->mm or not. > >In other words, we might want to allow collapse if workload itself asks >for it, but not for khugepaged or *external* process_madvise(MADV_COLLAPSE). >is_khugepaged is not the right proxy. Right, IIUC, we have two options so far: A) Conservatively reject collapse for any UFFD-minor-registered VMA, as this patch does. B) Allow collapse when the workload itself asks for it, but reject khugepaged and external process_madvise(MADV_COLLAPSE). Maybe there is a third option as well. Would be good to hear more thoughts on the expected semantics, especially from the maintainers, before deciding what the check should look like. @James, thanks for spinning v2 so quickly :) Please don't send another revision until we settle the expected semantics here. Hope you don't mind giving us a little time to sort this out :D Cheers, Lance