Re: [syzbot] [mm?] WARNING in unmap_page_range (2)

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It should be fine, as:

static void make_uffd_wp_pte(struct vm_area_struct *vma,
			     unsigned long addr, pte_t *pte)
{
	pte_t ptent = ptep_get(pte);

#ifndef CONFIG_USERFAULTFD_

	if (pte_present(ptent)) {
		pte_t old_pte;

		old_pte = ptep_modify_prot_start(vma, addr, pte);
		ptent = pte_mkuffd_wp(ptent);
		ptep_modify_prot_commit(vma, addr, pte, old_pte, ptent);
	} else if (is_swap_pte(ptent)) {
		ptent = pte_swp_mkuffd_wp(ptent);
		set_pte_at(vma->vm_mm, addr, pte, ptent);
	} else {                                      <----------------- this must be pte_none() already
		set_pte_at(vma->vm_mm, addr, pte,
			   make_pte_marker(PTE_MARKER_UFFD_WP));
	}
}

Indeed! Is pte_swp_mkuffd_wp() reasonable for pte markers? I rememebr that we don't support multiple markers yet, so it might be good enough.



2) We get the error on arm64, which does *not* support uffd-wp. Do we
    maybe end up calling make_uffd_wp_pte() and place a pte marker, even
    though we don't have CONFIG_PTE_MARKER_UFFD_WP?


static inline bool pte_marker_entry_uffd_wp(swp_entry_t entry)
{
#ifdef CONFIG_PTE_MARKER_UFFD_WP
	return is_pte_marker_entry(entry) &&
	    (pte_marker_get(entry) & PTE_MARKER_UFFD_WP);
#else
	return false;
#endif
}

Will always return false without CONFIG_PTE_MARKER_UFFD_WP.

But make_uffd_wp_pte() might just happily place an entry. Hm.


The following might fix the problem:


[...]


I'd like to double check with Muhammad (as I didn't actually follow his
work in the latest versions.. quite a lot changed), but I _think_
fundamentally we missed something important in the fast path, and I think
it applies even to archs that support uffd..

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index e91085d79926..3b81baabd22a 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -2171,7 +2171,8 @@ static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
                 return 0;
         }

-       if (!p->vec_out) {
+       if (!p->vec_out &&
+           (p->arg.flags & PM_SCAN_WP_MATCHING))

Ouch, yes. So that's the global fence I was wondering where to find it.

--
Cheers,

David / dhildenb





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