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Fri, 23 Oct 2026 05:15:18 -0700 (PDT) Message-ID: <538f7c6f-3e16-4ab9-9b79-232b34fb4d3e@redhat.com> Date: Fri, 23 Oct 2026 08:15:17 -0400 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [REGRESSION] mremap() fails VMA expansion when VM_MIXEDMAP is set To: "Lorenzo Stoakes (ARM)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , David Hildenbrand , Jakub Matena , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261023023106.515365-1-luizcap@redhat.com> Content-Language: en-US From: Luiz Capitulino In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/23/26 4:18 AM, Lorenzo Stoakes (ARM) wrote: > I'll look into this but obviously we won't make mixed map regions mergeable (nor > were they ever), that's completely broken. > > The most recent version of the mm code (which is going to be send to Linus soon > in this merge window) documents why: > > /** > * vma_flags_can_merge() - Do the specified VMA flags permit the VMA to be > * merged with another? > * @flags: The VMA flags to test. > * Returns: true if the flags permit merging, false otherwise. > */ > static inline bool vma_flags_can_merge(const vma_flags_t *flags) > { > /* > * VMA merging assumes that a VMA's flags and fields completely describe > * its state, which is only the case for mm-backed mappings. > * > * Mappings which are not mm-managed may have established state upon > * mapping not embodied in any attribute of the VMA. > * > * Additionally, private (CoW) PFN maps encode the source PFN of the > * range in vma->vm_pgoff, which may otherwise cause spurious merges. > */ > if (!vma_flags_is_mm_backed(flags)) > return false; > /* > * hugetlb mappings contain per-VMA state, so their flags and fields do > * not completely describe them. > */ > return !vma_flags_is_hugetlb(flags); > } > > I'll dig in + get back to you. Thanks Lorenzo, I really appreciate it. > > -- > Cheers, Lorenzo