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Date: Mon, 27 Feb 2023 11:16:53 -0800
From: Mike Kravetz <mike.kravetz@...cle.com>
To: James Houghton <jthoughton@...gle.com>
Cc: Muchun Song <songmuchun@...edance.com>,
Peter Xu <peterx@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
David Rientjes <rientjes@...gle.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Mina Almasry <almasrymina@...gle.com>,
Zach O'Keefe <zokeefe@...gle.com>,
Manish Mishra <manish.mishra@...anix.com>,
Naoya Horiguchi <naoya.horiguchi@....com>,
"Dr . David Alan Gilbert" <dgilbert@...hat.com>,
"Matthew Wilcox (Oracle)" <willy@...radead.org>,
Vlastimil Babka <vbabka@...e.cz>,
Baolin Wang <baolin.wang@...ux.alibaba.com>,
Miaohe Lin <linmiaohe@...wei.com>,
Yang Shi <shy828301@...il.com>,
Frank van der Linden <fvdl@...gle.com>,
Jiaqi Yan <jiaqiyan@...gle.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/46] hugetlb: add hugetlb_alloc_pmd and
hugetlb_alloc_pte
On 02/18/23 00:27, James Houghton wrote:
> These functions are used to allocate new PTEs below the hstate PTE. This
> will be used by hugetlb_walk_step, which implements stepping forwards in
> a HugeTLB high-granularity page table walk.
>
> The reasons that we don't use the standard pmd_alloc/pte_alloc*
> functions are:
> 1) This prevents us from accidentally overwriting swap entries or
> attempting to use swap entries as present non-leaf PTEs (see
> pmd_alloc(); we assume that !pte_none means pte_present and
> non-leaf).
> 2) Locking hugetlb PTEs can different than regular PTEs. (Although, as
> implemented right now, locking is the same.)
> 3) We can maintain compatibility with CONFIG_HIGHPTE. That is, HugeTLB
> HGM won't use HIGHPTE, but the kernel can still be built with it,
> and other mm code will use it.
>
> When GENERAL_HUGETLB supports P4D-based hugepages, we will need to
> implement hugetlb_pud_alloc to implement hugetlb_walk_step.
>
> Signed-off-by: James Houghton <jthoughton@...gle.com>
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index eeacadf3272b..9d839519c875 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -72,6 +72,11 @@ unsigned long hugetlb_pte_mask(const struct hugetlb_pte *hpte)
>
> bool hugetlb_pte_present_leaf(const struct hugetlb_pte *hpte, pte_t pte);
>
> +pmd_t *hugetlb_alloc_pmd(struct mm_struct *mm, struct hugetlb_pte *hpte,
> + unsigned long addr);
> +pte_t *hugetlb_alloc_pte(struct mm_struct *mm, struct hugetlb_pte *hpte,
> + unsigned long addr);
> +
> struct hugepage_subpool {
> spinlock_t lock;
> long count;
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6c74adff43b6..bb424cdf79e4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -483,6 +483,120 @@ static bool has_same_uncharge_info(struct file_region *rg,
> #endif
> }
>
> +/*
> + * hugetlb_alloc_pmd -- Allocate or find a PMD beneath a PUD-level hpte.
> + *
> + * This is meant to be used to implement hugetlb_walk_step when one must go to
> + * step down to a PMD. Different architectures may implement hugetlb_walk_step
> + * differently, but hugetlb_alloc_pmd and hugetlb_alloc_pte are architecture-
> + * independent.
> + *
> + * Returns:
> + * On success: the pointer to the PMD. This should be placed into a
> + * hugetlb_pte. @hpte is not changed.
> + * ERR_PTR(-EINVAL): hpte is not PUD-level
> + * ERR_PTR(-EEXIST): there is a non-leaf and non-empty PUD in @hpte
I often get this confused, should this really be 'non-leaf'? Because, ...
> + * ERR_PTR(-ENOMEM): could not allocate the new PMD
> + */
> +pmd_t *hugetlb_alloc_pmd(struct mm_struct *mm, struct hugetlb_pte *hpte,
> + unsigned long addr)
> +{
> + spinlock_t *ptl = hugetlb_pte_lockptr(hpte);
> + pmd_t *new;
> + pud_t *pudp;
> + pud_t pud;
> +
> + if (hpte->level != HUGETLB_LEVEL_PUD)
> + return ERR_PTR(-EINVAL);
> +
> + pudp = (pud_t *)hpte->ptep;
> +retry:
> + pud = READ_ONCE(*pudp);
> + if (likely(pud_present(pud)))
> + return unlikely(pud_leaf(pud))
> + ? ERR_PTR(-EEXIST)
> + : pmd_offset(pudp, addr);
... it seems we return -EEXIST in the pud_leaf case.
--
Mike Kravetz
> + else if (!pud_none(pud))
> + /*
> + * Not present and not none means that a swap entry lives here,
> + * and we can't get rid of it.
> + */
> + return ERR_PTR(-EEXIST);
> +
> + new = pmd_alloc_one(mm, addr);
> + if (!new)
> + return ERR_PTR(-ENOMEM);
> +
> + spin_lock(ptl);
> + if (!pud_same(pud, *pudp)) {
> + spin_unlock(ptl);
> + pmd_free(mm, new);
> + goto retry;
> + }
> +
> + mm_inc_nr_pmds(mm);
> + smp_wmb(); /* See comment in pmd_install() */
> + pud_populate(mm, pudp, new);
> + spin_unlock(ptl);
> + return pmd_offset(pudp, addr);
> +}
> +
> +/*
> + * hugetlb_alloc_pte -- Allocate a PTE beneath a pmd_none PMD-level hpte.
> + *
> + * See the comment above hugetlb_alloc_pmd.
> + */
> +pte_t *hugetlb_alloc_pte(struct mm_struct *mm, struct hugetlb_pte *hpte,
> + unsigned long addr)
> +{
> + spinlock_t *ptl = hugetlb_pte_lockptr(hpte);
> + pgtable_t new;
> + pmd_t *pmdp;
> + pmd_t pmd;
> +
> + if (hpte->level != HUGETLB_LEVEL_PMD)
> + return ERR_PTR(-EINVAL);
> +
> + pmdp = (pmd_t *)hpte->ptep;
> +retry:
> + pmd = READ_ONCE(*pmdp);
> + if (likely(pmd_present(pmd)))
> + return unlikely(pmd_leaf(pmd))
> + ? ERR_PTR(-EEXIST)
> + : pte_offset_kernel(pmdp, addr);
> + else if (!pmd_none(pmd))
> + /*
> + * Not present and not none means that a swap entry lives here,
> + * and we can't get rid of it.
> + */
> + return ERR_PTR(-EEXIST);
> +
> + /*
> + * With CONFIG_HIGHPTE, calling `pte_alloc_one` directly may result
> + * in page tables being allocated in high memory, needing a kmap to
> + * access. Instead, we call __pte_alloc_one directly with
> + * GFP_PGTABLE_USER to prevent these PTEs being allocated in high
> + * memory.
> + */
> + new = __pte_alloc_one(mm, GFP_PGTABLE_USER);
> + if (!new)
> + return ERR_PTR(-ENOMEM);
> +
> + spin_lock(ptl);
> + if (!pmd_same(pmd, *pmdp)) {
> + spin_unlock(ptl);
> + pgtable_pte_page_dtor(new);
> + __free_page(new);
> + goto retry;
> + }
> +
> + mm_inc_nr_ptes(mm);
> + smp_wmb(); /* See comment in pmd_install() */
> + pmd_populate(mm, pmdp, new);
> + spin_unlock(ptl);
> + return pte_offset_kernel(pmdp, addr);
> +}
> +
> static void coalesce_file_region(struct resv_map *resv, struct file_region *rg)
> {
> struct file_region *nrg, *prg;
> --
> 2.39.2.637.g21b0678d19-goog
>
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