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KVM: arm64: Top up stage-2 memcache for dirty logging faults

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Dirty logging forces new stage-2 mappings to page size but does not
always split an existing block first (eager splitting is best effort
and off by default). A non-write permission fault on such a block, an
instruction fetch, still needs a page-table page to split it, but the
top-up is gated on write faults.

With the cache empty, kvm_mmu_memory_cache_alloc() hits its
guest-triggerable WARN_ON(!nobjs) and falls back to a GFP_ATOMIC
allocation under mmu_lock, with a BUG_ON() if that fails.

Top up the memcache for any permission fault while dirty logging is
active.

Fixes: 6f745f1bb5 ("KVM: arm64: Convert user_mem_abort() to generic page-table API")
Link: https://lore.kernel.org/all/20260623165634.699011F000E9@smtp.kernel.org/
Signed-off-by: Bradley Morgan <include@grrlz.net>
[tabba: reword the commit message for the generic, non-pKVM failure mode]
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Tested-by: Bradley Morgan <include@grrlz.net> # On pixel 7, Android 17 CP2A.260705.006
Test: Bradley Morgan <include@grrlz.net> # On QEMU arm64 host
Link: https://patch.msgid.link/20260717130317.1953574-4-fuad.tabba@linux.dev
Signed-off-by: Oliver Upton <oupton@kernel.org>
This commit is contained in:
Bradley Morgan 2026-07-17 14:03:13 +01:00 committed by Oliver Upton
parent 6bf0215c56
commit 56fd1fc526
1 changed files with 2 additions and 3 deletions
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5
arch/arm64/kvm/mmu.c
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@ -2113,14 +2113,13 @@ static int user_mem_abort(const struct kvm_s2_fault_desc *s2fd)
* Permission faults just need to update the existing leaf entry,
* and so normally don't require allocations from the memcache. The
* only exception to this is when dirty logging is enabled at runtime
* and a write fault needs to collapse a block entry into a table.
* and a fault needs to collapse a block entry into a table.
* Under pKVM a permission fault can also collapse pages into a block,
* which needs a fresh mapping object, and the hypervisor requires the
* min-pages memcache even when the install allocates nothing.
*/
memcache = get_mmu_memcache(s2fd->vcpu);
if (!perm_fault || (memslot_is_logging(s2fd->memslot) &&
kvm_is_write_fault(s2fd->vcpu)) ||
if (!perm_fault || memslot_is_logging(s2fd->memslot) ||
is_protected_kvm_enabled()) {
ret = topup_mmu_memcache(s2fd->vcpu, memcache);
if (ret)