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4JeLXqFffOH1ki6gz1lkqzKK1wSLaqCyZKQZkaXeVPU/g/TiHrskOIgw1ydxzYzqoGY8a6gO1Wy nO2hd4vrbPkr5bjetQwMFR56+FK+7FgJU2t6EXA/SXXYTkN22NcUPi3d7eIKr1RjKHTgdqjyvHc NLcF0hBvqi8MoAN8M/x32jqmRTRDg521gPWs1MGEChRVa9qqYTxwJn6XyYEoIdgRhY1MEXrQmw7 IG0fA3dRy+S0rmTXSIe8dfvv6lcFvJW4jbJNJ1T4jwrc6RQBAc5LKasjaHDiCISHokmLdwyLp4K SageefD65FhJPpdo8plwbEhikVYIeVaiW5l24kIeCtR2FtkzsLgyn7+7OOjL1BdUM0J3dkNFFLD udhTWfZ4NTUIJVCFxi4FQzVMfG4t7VKvRRsW2vAI= X-Authority-Analysis: v=2.4 cv=aID9aL9m c=1 sm=1 tr=0 ts=695ca5b5 b=1 cx=c_pps a=e1sVV491RgrpLwSTMOnk8w==:117 a=e1sVV491RgrpLwSTMOnk8w==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=kj9zAlcOel0A:10 a=vUbySO9Y5rIA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=yPCof4ZbAAAA:8 a=VwQbUJbxAAAA:8 a=pFG2RB08WNx_kCNatiQA:9 a=CjuIK1q_8ugA:10 cc=ntf awl=host:13654 X-Proofpoint-GUID: 3Wa2mtnVwluCECtcKt2Et3TjQ1xQBN62 On Mon, Jan 05, 2026 at 08:11:49PM +0000, Lorenzo Stoakes wrote: > The is_mergeable_anon_vma() function uses vmg->middle as the source > VMA. However when merging a new VMA, this field is NULL. > > In all cases except mremap(), the new VMA will either be newly established > and thus lack an anon_vma, or will be an expansion of an existing VMA thus > we do not care about whether VMA is CoW'd or not. > > In the case of an mremap(), we can end up in a situation where we can > accidentally allow an unfaulted/faulted merge with a VMA that has been > forked, violating the general rule that we do not permit this for reasons > of anon_vma lock scalability. > > Now we have the ability to be aware of the fact we are copying a VMA and > also know which VMA that is, we can explicitly check for this, so do so. > > This is pertinent since commit 879bca0a2c4f ("mm/vma: fix incorrectly > disallowed anonymous VMA merges"), as this patch permits unfaulted/faulted > merges that were previously disallowed running afoul of this issue. > > While we are here, vma_had_uncowed_parents() is a confusing name, so make > it simple and rename it to vma_is_fork_child(). > > Signed-off-by: Lorenzo Stoakes > Fixes: 879bca0a2c4f ("mm/vma: fix incorrectly disallowed anonymous VMA merges") > Cc: stable@kernel.org > --- LGTM, so: Reviewed-by: Harry Yoo -- Cheers, Harry / Hyeonggon