Messages in this thread | | | Date | Sat, 30 Apr 2022 11:22:33 +0800 | Subject | Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping | From | Baolin Wang <> |
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On 4/30/2022 4:02 AM, Gerald Schaefer wrote: > On Fri, 29 Apr 2022 16:14:43 +0800 > Baolin Wang <baolin.wang@linux.alibaba.com> 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@linux.alibaba.com> >> --- >> 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().
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