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

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Message-ID: <fb6da8ab-01c7-4c5d-b638-d82c9e4e1423@amazon.com>
Date: Wed, 14 Jan 2026 13:56:10 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
To: Dave Hansen <dave.hansen@...el.com>, "Kalyazin, Nikita"
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Subject: Re: [PATCH v8 05/13] KVM: guest_memfd: Add flag to remove from direct
 map



On 05/12/2025 17:30, Dave Hansen wrote:
> 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.

Done in v9, thanks.