Re: [PATCH v3 01/18] x86/PoD: map one page at a time in p2m_pod_zero_check()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.10.2026 12:40, George Dunlap wrote:
> p2m_pod_zero_check() maps every page of its batch that passes its
> checks, up to POD_SWEEP_STRIDE (16) of them, and keeps them mapped
> across the p2m lookups and updates which follow, each of which maps
> page-table pages of its own.  Its callers hold mappings too: the p2m
> lookup which found the entry to populate still has its page-table page
> mapped, and a PV mmu_update() the L1 page it is writing.
> 
> map_domain_page() cannot always provide that many at once.  It uses the
> mapcache on behalf of PV vCPUs only: in debug builds for every page, in
> release builds for pages beyond the reach of the directmap in PV guests'
> page-tables (5TiB, 3.5TiB with CONFIG_BIGMEM).  The mapcache has
> MAPCACHE_VCPU_ENTRIES (16) slots per vCPU of the domain, shared by its
> vCPUs, and a mapping which finds no slot free or reclaimable hits the
> BUG_ON() in map_domain_page(): the host crashes.
> 
> The sweep runs in the context of whichever vCPU populates a PoD entry
> while the domain's PoD cache is empty.  The guest's own vCPUs, being
> HVM, map through the directmap; a PV vCPU gets there when its domain
> maps or copies the guest's memory: a device model in dom0 or in a stub
> domain, a backend doing grant copies.  A PV domain with a single vCPU,
> as a device-model stub domain or a dom0 booted with dom0_max_vcpus=1
> is, has 16 slots, and a sweep over 16 candidate pages needs more.
> 
> The sweep has mapped its whole batch at once since 9ad9a1609fbe ("PoD
> memory 5/9: emergency scan"), when x86-64 reached all memory through the
> directmap.  The per-vCPU budget came with the mapcache's return to
> x86-64 in 4b28bf6ae90b ("x86: re-introduce map_domain_page() et al").
> 
> Map one page at a time instead, as p2m_pod_zero_check_superpage()
> already does: map a page for the quick check and unmap it again, and
> map it anew for the full check after the p2m update and the TLB flush.
> The checks and their order are unchanged.  Each page is mapped twice,
> which costs little next to scanning it, and the error paths no longer
> have mappings to undo.
> 
> Fixes: 4b28bf6ae90b ("x86: re-introduce map_domain_page() et al")
> Assisted-by: Claude Code:claude-opus-5-5
> Signed-off-by: George Dunlap <gwd@xxxxxxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>