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Tue, 3 May 2022 10:03:46 GMT Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6C36DAE057; Tue, 3 May 2022 10:03:46 +0000 (GMT) Received: from d06av26.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 25EF4AE053; Tue, 3 May 2022 10:03:45 +0000 (GMT) Received: from thinkpad (unknown [9.171.25.226]) by d06av26.portsmouth.uk.ibm.com (Postfix) with SMTP; Tue, 3 May 2022 10:03:45 +0000 (GMT) Date: Tue, 3 May 2022 12:03:43 +0200 From: Gerald Schaefer To: Baolin Wang Cc: akpm@linux-foundation.org, mike.kravetz@oracle.com, catalin.marinas@arm.com, will@kernel.org, tsbogend@alpha.franken.de, James.Bottomley@HansenPartnership.com, deller@gmx.de, mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, ysato@users.sourceforge.jp, dalias@libc.org, davem@davemloft.net, arnd@arndb.de, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 3/3] mm: rmap: Fix CONT-PTE/PMD size hugetlb issue when unmapping Message-ID: <20220503120343.6264e126@thinkpad> In-Reply-To: <48a05075-a323-e7f1-9e99-6c0d106eb2cb@linux.alibaba.com> References: <20220429220214.4cfc5539@thinkpad> <20220502160232.589a6111@thinkpad> <48a05075-a323-e7f1-9e99-6c0d106eb2cb@linux.alibaba.com> X-Mailer: Claws Mail 4.0.0 (GTK+ 3.24.31; 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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, 3 May 2022 10:19:46 +0800 Baolin Wang wrote: > > > On 5/2/2022 10:02 PM, Gerald Schaefer wrote: > > On Sat, 30 Apr 2022 11:22:33 +0800 > > Baolin Wang wrote: > > > >> > >> > >> On 4/30/2022 4:02 AM, Gerald Schaefer wrote: > >>> On Fri, 29 Apr 2022 16:14:43 +0800 > >>> Baolin Wang 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 > >>>> --- > >>>> 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). 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). > > /* Set the dirty flag on the folio now the pte is gone. */ > if (pte_dirty(pteval)) > folio_mark_dirty(folio); Ok, that should work fine, huge_ptep_clear_flush() will return a pteval properly de-referenced and converted with huge_ptep_get(), and that would contain the hugetlb pmd/pud dirty information. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel