Re: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb

[Peter Xu]

On Thu, Jan 05, 2023 at 10:18:33AM +0000, James Houghton wrote:
> +	/*
> +	 * Grab the VMA lock and mapping sem for writing. This will prevent
> +	 * concurrent high-granularity page table walks, so that we can safely
> +	 * collapse and free page tables.
> +	 *
> +	 * This is the same locking that huge_pmd_unshare requires.
> +	 */
> +	hugetlb_vma_lock_write(vma);
> +	i_mmap_lock_write(vma->vm_file->f_mapping);

One thing I just noticed - do we need the mmap write lock here?  I don't
quickly see what stops another thread from having the mmap read and walking
upon the pgtables being collapsed.

Thanks,

-- 
Peter Xu