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Date: Fri, 16 Jan 2026 15:00:26 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
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Subject: Re: [PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct
 map



On 16/01/2026 00:00, Edgecombe, Rick P wrote:
> On Wed, 2026-01-14 at 13:46 +0000, 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 folio_zap_direct_map() in kvm_gmem_folio_zap_direct_map().
>> +      * Thus folio_restore_direct_map() here only updates prot bits.
>> +      */
>> +     if (kvm_gmem_folio_no_direct_map(folio)) {
>> +             WARN_ON_ONCE(folio_restore_direct_map(folio));
>> +             folio->private = (void *)((u64)folio->private & ~KVM_GMEM_FOLIO_NO_DIRECT_MAP);
>> +     }
>> +}
>> +
> 
> Does this assume the folio would not have been split after it was zapped? As in,
> if it was zapped at 2MB granularity (no 4KB direct map split required) but then
> restored at 4KB (split required)? Or it gets merged somehow before this?

AFAIK it can't be zapped at 2MB granularity as the zapping code will 
inevitably cause splitting because guest_memfd faults occur at the base 
page granularity as of now.

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