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Message-ID: <4c24b891-04ce-2608-79d2-a75dc236533f@redhat.com>
Date: Tue, 23 Aug 2022 10:29:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>,
akpm@...ux-foundation.org, songmuchun@...edance.com,
mike.kravetz@...cle.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] mm/hugetlb: fix races when looking up a CONT-PTE
size hugetlb page
On 23.08.22 09:50, 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.
>
> So when looking up a CONT-PTE size hugetlb page by follow_page(), it
> will use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE
> size hugetlb in follow_page_pte(). However this pte entry lock is incorrect
> for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to
> get the correct lock, which is mm->page_table_lock.
>
> That means the pte entry of the CONT-PTE size hugetlb under current
> pte lock is unstable in follow_page_pte(), we can continue to migrate
> or poison the pte entry of the CONT-PTE size hugetlb, which can cause
> some potential race issues, and following pte_xxx() validation is also
> unstable in follow_page_pte(), even though they are under the 'pte lock'.
>
> Moreover we should use huge_ptep_get() to get the pte entry value of
> the CONT-PTE size hugetlb, which already takes into account the subpages'
> dirty or young bits in case we missed the dirty or young state of the
> CONT-PTE size hugetlb.
>
> To fix above issues, introducing a new helper follow_huge_pte() to look
> up a CONT-PTE size hugetlb page, which uses huge_pte_lock() to get the
> correct pte entry lock to make the pte entry stable, as well as
> supporting non-present pte handling.
>
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
> include/linux/hugetlb.h | 8 ++++++++
> mm/gup.c | 11 ++++++++++
> mm/hugetlb.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 72 insertions(+)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 3ec981a..d491138 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -207,6 +207,8 @@ struct page *follow_huge_addr(struct mm_struct *mm, unsigned long address,
> struct page *follow_huge_pd(struct vm_area_struct *vma,
> unsigned long address, hugepd_t hpd,
> int flags, int pdshift);
> +struct page *follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
> + pmd_t *pmd, int flags);
> struct page *follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> pmd_t *pmd, int flags);
> struct page *follow_huge_pud(struct mm_struct *mm, unsigned long address,
> @@ -312,6 +314,12 @@ static inline struct page *follow_huge_pd(struct vm_area_struct *vma,
> return NULL;
> }
>
> +static inline struct page *follow_huge_pte(struct vm_area_struct *vma,
> + unsigned long address, pmd_t *pmd, int flags)
> +{
> + return NULL;
> +}
> +
> static inline struct page *follow_huge_pmd(struct mm_struct *mm,
> unsigned long address, pmd_t *pmd, int flags)
> {
> diff --git a/mm/gup.c b/mm/gup.c
> index 3b656b7..87a94f5 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -534,6 +534,17 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> if (unlikely(pmd_bad(*pmd)))
> return no_page_table(vma, flags);
>
> + /*
> + * Considering PTE level hugetlb, like continuous-PTE hugetlb on
> + * ARM64 architecture.
> + */
> + if (is_vm_hugetlb_page(vma)) {
> + page = follow_huge_pte(vma, address, pmd, flags);
> + if (page)
> + return page;
> + return no_page_table(vma, flags);
> + }
> +
> ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
> pte = *ptep;
> if (!pte_present(pte)) {
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6c00ba1..cf742d1 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6981,6 +6981,59 @@ struct page * __weak
> return NULL;
> }
>
> +/* Support looking up a CONT-PTE size hugetlb page. */
> +struct page * __weak
> +follow_huge_pte(struct vm_area_struct *vma, unsigned long address,
> + pmd_t *pmd, int flags)
> +{
> + struct mm_struct *mm = vma->vm_mm;
> + struct hstate *hstate = hstate_vma(vma);
> + unsigned long size = huge_page_size(hstate);
> + struct page *page = NULL;
> + spinlock_t *ptl;
> + pte_t *ptep, pte;
> +
> + /*
> + * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> + * follow_hugetlb_page().
> + */
> + if (WARN_ON_ONCE(flags & FOLL_PIN))
> + return NULL;
> +
> + ptep = huge_pte_offset(mm, address, size);
> + if (!ptep)
> + return NULL;
> +
> +retry:
> + ptl = huge_pte_lock(hstate, mm, ptep);
> + pte = huge_ptep_get(ptep);
> + if (pte_present(pte)) {
> + page = pte_page(pte);
> + if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> + page = NULL;
> + goto out;
> + }
> + } else {
> + if (!(flags & FOLL_MIGRATION)) {
> + page = NULL;
> + goto out;
> + }
> +
> + if (is_hugetlb_entry_migration(pte)) {
> + spin_unlock(ptl);
> + __migration_entry_wait_huge(ptep, ptl);
> + goto retry;
> + }
> + /*
> + * hwpoisoned entry is treated as no_page_table in
> + * follow_page_mask().
> + */
> + }
> +out:
> + spin_unlock(ptl);
> + return page;
> +}
> +
> struct page * __weak
> follow_huge_pmd(struct mm_struct *mm, unsigned long address,
> pmd_t *pmd, int flags)
Can someone explain why:
* follow_page() goes via follow_page_mask() for hugetlb
* __get_user_pages() goes via follow_hugetlb_page() and never via
follow_page_mask() for hugetlb?
IOW, why can't we make follow_page_mask() just not handle hugetlb and
route everything via follow_hugetlb_page() -- we primarily only have to
teach it to not trigger faults.
What's the reason that this hugetlb code has to be overly complicated?
--
Thanks,
David / dhildenb
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