Re: [PATCH v6 02/19] selftests/mm: skip collapse_compound_extreme() where the PMD is too large

[Zenghui Yu <zenghui.yu@linux.dev>]

Hi Kiryl,

On 9/19/26 8:24 AM, Kiryl Shutsemau wrote:
> From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> 
> collapse_compound_extreme() builds a PTE table full of distinct PTE-mapped
> compound pages by cycling hpage_pmd_nr fault-time THPs through mremap.  It
> therefore needs hpage_pmd_nr PMD-order allocations in a row.  That is fine
> at a 2M PMD (4K base pages) or a 32M one (16K).  A 512M PMD -- arm64 with
> 64K base pages -- makes each of those an order-13 allocation, which the
> allocator cannot reliably hand out even once, let alone 8192 times.
> 
> The failure is not a quiet one: the case calls ksft_exit_fail_msg(), so the
> whole binary stops and every case after it is lost.
> 
> Skip the case where the PMD is larger than 32M.  The MADV_COLLAPSE cases
> still cover PMD-order collapse on those configurations, and 4K and 16K
> PMDs are unaffected.

collapse_compound_extreme() failed on an arm64 virtual machines with 16k
pages.  The VM has 8GB of memory.  Not sure if this is the same issue that
this patch attempts to address.

[root@localhost mm]# ./khugepaged

[ snipped ]

# 
# Run test: collapse_full_of_compound (madvise:anon)
# Allocate huge page... OK
# Split huge page leaving single PTE page table full of compound pages... OK
# Collapse PTE table full of compound pages... OK
ok 76 collapse_full_of_compound
# 
# Run test: collapse_compound_extreme (khugepaged:anon)
# Construct PTE page table full of different PTE-mapped compound pages
Bail out! Failed to allocate huge page
# 1 skipped test(s) detected. Consider enabling relevant config options to improve coverage.
# Planned tests != run tests (91 != 76)
# Totals: pass:75 fail:0 xfail:0 xpass:0 skip:1 error:0

Thanks,
Zenghui