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Howlett" , Vlastimil Babka , Jann Horn , Pedro Falcato , Jakub =?utf-8?Q?Mat=C4=9Bna?= Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Luiz Capitulino , stable@vger.kernel.org Subject: Re: [PATCH] mm/vma: fix expansion of VMA_MIXEDMAP_BIT VMAs Message-ID: References: <20261023-vma-expansion-fix-v1-1-6142baf1f46b@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261023-vma-expansion-fix-v1-1-6142baf1f46b@kernel.org> X-Rspam-User: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 1CEBD180004 X-Spam-Status: No, score=-11.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Stat-Signature: hc45xzf3mzaxsysfw8a8xhxf3aeu8ysx X-HE-Tag: 1792776664-295492 X-HE-Meta: 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 dFVtuWdC f+RLUJr81/K9bW1i0BYJCCQK9bQW4uK4c2vTaW1lNExM88zeCBMSuZG7xulcypSL68HHEp6dbVR+WeB/3bRsElLxDA4bR1Yc6737jAv+6ZsGQ2+y5w6gRzt7HNlUxCHwtvCELOWdjms/Vi4JiCnigtFdhou0qhQgfA2M3/9VSg+M4YNbFknug50erDTbgAIASngMIFXWItqlyvESepj7xnlC+tiseUXCclrscDyScFqQSOBVV83RXl3wvvtGPgn8QidIlV/AQfcyukaHnIyfZS5jn453EcxuO7V2/UnMJZCCquSkNqxbJHdNt4jYVci6XtTEPsmuv9URCWtmYLok0pYVrzQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Fri, Oct 23, 2026 at 04:17:31PM +0100, Lorenzo Stoakes (ARM) wrote: > Commit ca3d76b0aa80 ("mm: add merging after mremap resize") combined > merging with expansion of mremap()'d VMAs, however it also unintentionally > disallowed the expansion of unmergeable VMAs. > > Previously it was possible to expand a mixed mapping which mremap() had > already ensured was a legal expansion. > > This is clearly seen in the mremap() logic, as it checks > vma_is_fixed_mapping() to determine legality of an expansion rather than > vma_can_merge(). > > Additionally, copy_vma() allows for a failed merge to result in a new VMA > being instantiated if MREMAP_MAYMOVE is specified. > > However, this is not attempted, as a failure in vma_merge_extend() is > assumed to be an -ENOMEM failure. Just to note that VMA_MIXEDMAP_BIT VMAs can _currently_ be expanded if not attempted to be expanded in-place (e.g. MREMAP_FIXED, or if not enough space to expand into). So this patch doesn't re-add the ability to do that per se, it just fixes one specific means of doing what is already a valid operation. VMA_MIXEDMAP _expanding_ isn't really a problem - if no fault handler is installed, faults in the expanded region will SIGBUS, and if one exists it'll be invoked correctly. This doesn't work for VMA_PFNMAP_BIT because the page offset might be co-opted for CoW (!) and thus is unreliable. And it seems some users do, at least in theory, require it, even if they are using older kernel revisions :) > Since vma_merge_extend() is used exclusively by mremap() to perform a merge > and/or expansion, resolve the issue by invoking vma_expand() for > unmergeable VMAs there. > > Explicitly assert that the VMA is both non-fixed and a mixed mapping. > This provides double insurance against broken changes in future. > > Note that khugepaged_enter_vma() (which vma_merge_new_range() runs) is > redundant in this case, as a mixed mapping is not subject to khugepaged. > > Also update the VMA userland tests to add a required predicate. > > Reported-by: Luiz Capitulino > Closes: https://lore.kernel.org/all/20261023023106.515365-1-luizcap@redhat.com/ > Fixes: ca3d76b0aa80 ("mm: add merging after mremap resize") > Cc: stable@vger.kernel.org > Signed-off-by: Lorenzo Stoakes (ARM) > --- > Confirmed that this fixes the repro and passes mm selftests and the VMA > userland tests. -- Cheers, Lorenzo