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Fri, 28 Aug 2026 09:07:08 -0400 (EDT) Date: Fri, 28 Aug 2026 14:07:07 +0100 From: Kiryl Shutsemau To: Lance Yang Cc: jthoughton@google.com, 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 Message-ID: References: <20260828005004.2870750-1-jthoughton@google.com> <20260828094703.11081-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260828094703.11081-1-lance.yang@linux.dev> X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 1A364100013 X-Stat-Signature: 4mkbs5humoyyn1ifomcc6pw7w18ucgce X-HE-Tag: 1787922435-534943 X-HE-Meta: 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 8xE5LFLA nDh8FSeHZNfRGzgqmjPe0lwJWy9Xp0ZY361ihI06BIaiFCfhTiMq/ZVUFHI3Tz8Y27bFg6qYVsQ/OWYEEoK9rX633ZSP9KkP7mB4qEq1KgdtVdJEgM/dLxEYXlddrHtwYYNRrmX2ZiU1zRFI7WSe0pscX24andivfHP5bgz2HKOGwRRWJMEnFV+dkqhGF58XGCyRgB5jq3G/7hVCHEuPjOPAJTcPURg5xCWb9X4/QblJj473zOA4vsPtAZapEUifQCuuqaUGMmTK68g1gjBn+hT7VbA== 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 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. -- Kiryl Shutsemau / Kirill A. Shutemov