linux-kernel - Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically split/reconstruct HugeTLB page

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Message-ID: <Z+6AGxEvBRFkN5mN@yzhao56-desk.sh.intel.com>
Date: Thu, 3 Apr 2025 20:33:31 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [RFC PATCH 39/39] KVM: guest_memfd: Dynamically
 split/reconstruct HugeTLB page

On Tue, Sep 10, 2024 at 11:44:10PM +0000, Ackerley Tng wrote:
> +/*
> + * Allocates and then caches a folio in the filemap. Returns a folio with
> + * refcount of 2: 1 after allocation, and 1 taken by the filemap.
> + */
> +static struct folio *kvm_gmem_hugetlb_alloc_and_cache_folio(struct inode *inode,
> +							    pgoff_t index)
> +{
> +	struct kvm_gmem_hugetlb *hgmem;
> +	pgoff_t aligned_index;
> +	struct folio *folio;
> +	int nr_pages;
> +	int ret;
> +
> +	hgmem = kvm_gmem_hgmem(inode);
> +	folio = kvm_gmem_hugetlb_alloc_folio(hgmem->h, hgmem->spool);
> +	if (IS_ERR(folio))
> +		return folio;
> +
> +	nr_pages = 1UL << huge_page_order(hgmem->h);
> +	aligned_index = round_down(index, nr_pages);
Maybe a gap here.

When a guest_memfd is bound to a slot where slot->base_gfn is not aligned to
2M/1G and slot->gmem.pgoff is 0, even if an index is 2M/1G aligned, the
corresponding GFN is not 2M/1G aligned.

However, TDX requires that private huge pages be 2M aligned in GFN.

> +	ret = kvm_gmem_hugetlb_filemap_add_folio(inode->i_mapping, folio,
> +						 aligned_index,
> +						 htlb_alloc_mask(hgmem->h));
> +	WARN_ON(ret);
> +
>  	spin_lock(&inode->i_lock);
>  	inode->i_blocks += blocks_per_huge_page(hgmem->h);
>  	spin_unlock(&inode->i_lock);
>  
> -	return page_folio(requested_page);
> +	return folio;
> +}