Re: [PATCH] mm: clear pmd in pmdp_invalidate()

[Guilherme Giacomo Simoes <trintaeoitogc@gmail.com>]

"David Hildenbrand (Arm)" <david@kernel.org> wrote:
> > when attempt to do a madvise in a chunk of memory allocated by mmap, the
> > folio_split is called to split a portion of memory, and despite the path
> > leads to pmd_mkinvalid() they do not clear the pmd and clearer wording,
> > leading to a BUG_ON inside page_table_check_set() because the map count
> > is more than 1.
> > 
> > Call the page_table_check_pmd_clear() inside pmdp_invalidate() generic
> > function to ensure that the pmd will have they map count decrement.
> > 
> 
> The subject + description is rather confusing: you are not "clearing the pmd",
> you tell page-table-check that the PMD was cleared.
> 
> This is all about apge-table-check.
Yes, you are right, i'm not clearing the pmd, I will improve this.

> 
> Can you post the actual kernel splat you ran into? Or is this generated by an LLM?
Of course, the stack trace of error is (already decoded):
page_table_check_set (mm/page_table_check.c:118)
__page_table_check_ptes_set (mm/page_table_check.c:223)
remove_migration_pte (include/linux/page_table_check.h:83 arch/riscv/include/asm/pgtable.h:642 mm/migrate.c:448)
rmap_walk_anon (mm/rmap.c:3053)
rmap_walk_locked (mm/rmap.c:3169 mm/rmap.c:3164)
remove_migration_ptes (mm/migrate.c:483)
__folio_split (mm/huge_memory.c:3640 mm/huge_memory.c:4289)
split_folio_to_list (mm/huge_memory.c:4435 include/linux/huge_mm.h:416 mm/huge_memory.c:4499)
madvise_cold_or_pageout_pte_range (mm/madvise.c:420)
walk_pgd_range (mm/pagewalk.c:147 mm/pagewalk.c:237 mm/pagewalk.c:278 mm/pagewalk.c:319)
__walk_page_range (mm/pagewalk.c:427)
walk_page_range_vma_unsafe (mm/pagewalk.c:737)
walk_page_range_vma (mm/pagewalk.c:747)
madvise_cold (mm/madvise.c:594 mm/madvise.c:614)
madvise_vma_behavior (mm/madvise.c:1364)
madvise_walk_vmas (mm/madvise.c:1721)
madvise_do_behavior (mm/madvise.c:1916)
do_madvise (mm/madvise.c:2014)
__riscv_sys_madvise (mm/madvise.c:2023 mm/madvise.c:2021 mm/madvise.c:2021)
syscall_handler (arch/riscv/include/asm/syscall.h:112)
do_trap_ecall_u (arch/riscv/kernel/traps.c:341)
handle_exception (arch/riscv/kernel/entry.S:237)

And the repro.c that I writed to reproduce bug:
```
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <errno.h>
#include <linux/mman.h>

#ifndef MADV_COLD
#define MADV_COLD 20
#endif

#define HPAGE_PMD_SIZE (2 * 1024 * 1024)  // 2MB huge page on RISC-V

int main(void)
{
	char *addr;
	size_t len = HPAGE_PMD_SIZE * 2;  // 4MB - two huge pages
	int ret;

	printf("Testing page_table_check bug with madvise(MADV_COLD)...\n");
	printf("Huge page size: %zu MB\n", HPAGE_PMD_SIZE / (1024*1024));

	// Fallback: try without MAP_HUGETLB (kernel will use THP)
	printf("starting THP...\n");
	addr = mmap(NULL, len, PROT_READ | PROT_WRITE,
		    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if (addr == MAP_FAILED) {
		perror("mmap failed");
		return 1;
	}
	// Enable THP for this mapping
	ret = madvise(addr, len, MADV_HUGEPAGE);
	if (ret) {
		perror("madvise(MADV_HUGEPAGE) failed");
		return 1;
	}

	printf("Allocated memory at %p\n", addr);

	// Write to the pages to ensure they're mapped
	memset(addr, 'A', len);
	memset(addr + HPAGE_PMD_SIZE, 'B', HPAGE_PMD_SIZE);

	// Touch pages to ensure huge page allocation
	for (size_t i = 0; i < len; i += HPAGE_PMD_SIZE) {
		addr[i] = 'X';
	}

	printf("Memory initialized, calling madvise(MADV_COLD) on partial range...\n");

	// Call madvise(MADV_COLD) on a partial range of the first huge page
	// This should trigger the huge page split and the bug
	ret = madvise(addr + 4096, HPAGE_PMD_SIZE - 8192, MADV_COLD);
	if (ret) {
		perror("madvise(MADV_COLD) failed");
		// Don't return - the kernel might have crashed
	}

	printf("madvise returned: %d (errno: %d)\n", ret, errno);

	// If we get here without kernel crash, the bug might be fixed
	// or the conditions weren't met
	printf("Test completed (if kernel didn't crash, bug may be fixed or not triggered)\n");

	munmap(addr, len);
	return 0;
}
```

I see this BUG here:
https://lore.kernel.org/all/69851c62.a00a0220.37c87e.002e.GAE@google.com/T/

Yes, I'd should mencioned "Reported-by: syzbot+227179d5a8a87e9df90d@syzkaller.appspotmail.com", my bad