Re: [REGRESSION] mremap() fails VMA expansion when VM_MIXEDMAP is set

[Luiz Capitulino <luizcap@redhat.com>]

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