Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping
From: Baolin Wang
Date: Fri Apr 29 2022 - 23:22:04 EST
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);
}
}
Please note that huge_ptep_get functions do not return valid PTEs on s390,
and such PTEs must never be set directly with set_pte_at(), but only with
set_huge_pte_at().
Background is that, for hugetlb pages, we are of course not really dealing
with PTEs at this level, but rather PMDs or PUDs, depending on hugetlb size.
On s390, the layout is quite different for PTEs and PMDs / PUDs, and
unfortunately the hugetlb code is not properly reflecting this by using
PMD or PUD types, like the THP code does.
So, as work-around, on s390, the huge_ptep_xxx functions will return
only fake PTEs, which must be converted again to a proper PMD or PUD,
before writing them to the page table, which is what happens in
set_huge_pte_at(), but not in set_pte_at().
Thanks for your explanation. As I said as above, I think we've already
handled the hugetlb with set_huge_swap_pte_at() in try_to_unmap_one().