Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping
From: Baolin Wang
Date: Tue May 03 2022 - 09:33:13 EST
On 5/3/2022 6:03 PM, Gerald Schaefer wrote:
On Tue, 3 May 2022 10:19:46 +0800
Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 5/2/2022 10:02 PM, Gerald Schaefer wrote:
On Sat, 30 Apr 2022 11:22:33 +0800
Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 4/30/2022 4:02 AM, Gerald Schaefer wrote:
On Fri, 29 Apr 2022 16:14:43 +0800
Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On some architectures (like ARM64), it can support CONT-PTE/PMD size
hugetlb, which means it can support not only PMD/PUD size hugetlb:
2M and 1G, but also CONT-PTE/PMD size: 64K and 32M if a 4K page
size specified.
When unmapping a hugetlb page, we will get the relevant page table
entry by huge_pte_offset() only once to nuke it. This is correct
for PMD or PUD size hugetlb, since they always contain only one
pmd entry or pud entry in the page table.
However this is incorrect for CONT-PTE and CONT-PMD size hugetlb,
since they can contain several continuous pte or pmd entry with
same page table attributes, so we will nuke only one pte or pmd
entry for this CONT-PTE/PMD size hugetlb page.
And now we only use try_to_unmap() to unmap a poisoned hugetlb page,
which means now we will unmap only one pte entry for a CONT-PTE or
CONT-PMD size poisoned hugetlb page, and we can still access other
subpages of a CONT-PTE or CONT-PMD size poisoned hugetlb page,
which will cause serious issues possibly.
So we should change to use huge_ptep_clear_flush() to nuke the
hugetlb page table to fix this issue, which already considered
CONT-PTE and CONT-PMD size hugetlb.
Note we've already used set_huge_swap_pte_at() to set a poisoned
swap entry for a poisoned hugetlb page.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/rmap.c | 34 +++++++++++++++++-----------------
1 file changed, 17 insertions(+), 17 deletions(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index 7cf2408..1e168d7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1564,28 +1564,28 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
break;
}
}
+ pteval = huge_ptep_clear_flush(vma, address, pvmw.pte);
Unlike in your patch 2/3, I do not see that this (huge) pteval would later
be used again with set_huge_pte_at() instead of set_pte_at(). Not sure if
this (huge) pteval could end up at a set_pte_at() later, but if yes, then
this would be broken on s390, and you'd need to use set_huge_pte_at()
instead of set_pte_at() like in your patch 2/3.
IIUC, As I said in the commit message, we will only unmap a poisoned
hugetlb page by try_to_unmap(), and the poisoned hugetlb page will be
remapped with a poisoned entry by set_huge_swap_pte_at() in
try_to_unmap_one(). So I think no need change to use set_huge_pte_at()
instead of set_pte_at() for other cases, since the hugetlb page will not
hit other cases.
if (PageHWPoison(subpage) && !(flags & TTU_IGNORE_HWPOISON)) {
pteval = swp_entry_to_pte(make_hwpoison_entry(subpage));
if (folio_test_hugetlb(folio)) {
hugetlb_count_sub(folio_nr_pages(folio), mm);
set_huge_swap_pte_at(mm, address, pvmw.pte, pteval,
vma_mmu_pagesize(vma));
} else {
dec_mm_counter(mm, mm_counter(&folio->page));
set_pte_at(mm, address, pvmw.pte, pteval);
}
}
OK, but wouldn't the pteval be overwritten here with
pteval = swp_entry_to_pte(make_hwpoison_entry(subpage))?
IOW, what sense does it make to save the returned pteval from
huge_ptep_clear_flush(), when it is never being used anywhere?
Please see previous code, we'll use the original pte value to check if
it is uffd-wp armed, and if need to mark it dirty though the hugetlbfs
is set noop_dirty_folio().
pte_install_uffd_wp_if_needed(vma, address, pvmw.pte, pteval);
Uh, ok, that wouldn't work on s390, but we also don't have
CONFIG_PTE_MARKER_UFFD_WP / HAVE_ARCH_USERFAULTFD_WP set, so
I guess we will be fine (for now).
OK.
Still, I find it a bit unsettling that pte_install_uffd_wp_if_needed()
would work on a potential hugetlb *pte, directly de-referencing it
instead of using huge_ptep_get().
The !pte_none(*pte) check at the beginning would be broken in the
hugetlb case for s390 (not sure about other archs, but I think s390
might be the only exception strictly requiring huge_ptep_get()
for de-referencing hugetlb *pte pointers).
Right, I think so too. I'll look at the uffd code in detail, seems need
another patch to fix the hugetlb for uffd. Thanks for your comments.