linux-kernel - Re: [PATCH v8 05/13] KVM: guest_memfd: Add flag to remove from direct map

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Message-ID: <51a059a1-f03a-4b43-8df6-d31fca09cce7@intel.com>
Date: Fri, 5 Dec 2025 09:30:09 -0800
From: Dave Hansen <dave.hansen@...el.com>
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Subject: Re: [PATCH v8 05/13] KVM: guest_memfd: Add flag to remove from direct
 map

On 12/5/25 08:58, Kalyazin, Nikita wrote:
> +static void kvm_gmem_folio_restore_direct_map(struct folio *folio)
> +{
> +	/*
> +	 * Direct map restoration cannot fail, as the only error condition
> +	 * for direct map manipulation is failure to allocate page tables
> +	 * when splitting huge pages, but this split would have already
> +	 * happened in set_direct_map_invalid_noflush() in kvm_gmem_folio_zap_direct_map().
> +	 * Thus set_direct_map_valid_noflush() here only updates prot bits.
> +	 */
> +	if (kvm_gmem_folio_no_direct_map(folio))
> +		set_direct_map_valid_noflush(folio_page(folio, 0), folio_nr_pages(folio),
> +					 true);
> +}

This is rather hopeful programming.

I can think of a number of ways that this assumption might become invalid.
This at *least* check for set_direct_map_valid_noflush() failures (or
whatever interface you end up using)?

A WARN_ON_ONCE() would be fine.