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Message-ID: <608934d4-466d-975e-6458-34a91ccb4669@redhat.com>
Date: Tue, 30 Aug 2022 10:11:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, inuxppc-dev@...ts.ozlabs.org,
linux-ia64@...r.kernel.org
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>,
"Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
Michael Ellerman <mpe@...erman.id.au>,
Muchun Song <songmuchun@...edance.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask
On 30.08.22 01:40, Mike Kravetz wrote:
> During discussions of this series [1], it was suggested that hugetlb
> handling code in follow_page_mask could be simplified. At the beginning
Feel free to use a Suggested-by if you consider it appropriate.
> of follow_page_mask, there currently is a call to follow_huge_addr which
> 'may' handle hugetlb pages. ia64 is the only architecture which provides
> a follow_huge_addr routine that does not return error. Instead, at each
> level of the page table a check is made for a hugetlb entry. If a hugetlb
> entry is found, a call to a routine associated with that entry is made.
>
> Currently, there are two checks for hugetlb entries at each page table
> level. The first check is of the form:
> if (p?d_huge())
> page = follow_huge_p?d();
> the second check is of the form:
> if (is_hugepd())
> page = follow_huge_pd().
BTW, what about all this hugepd stuff in mm/pagewalk.c?
Isn't this all dead code as we're essentially routing all hugetlb VMAs
via walk_hugetlb_range? [yes, all that hugepd stuff in generic code that
overcomplicates stuff has been annoying me for a long time]
>
> We can replace these checks, as well as the special handling routines
> such as follow_huge_p?d() and follow_huge_pd() with a single routine to
> handle hugetlb vmas.
>
> A new routine hugetlb_follow_page_mask is called for hugetlb vmas at the
> beginning of follow_page_mask. hugetlb_follow_page_mask will use the
> existing routine huge_pte_offset to walk page tables looking for hugetlb
> entries. huge_pte_offset can be overwritten by architectures, and already
> handles special cases such as hugepd entries.
>
> [1] https://lore.kernel.org/linux-mm/cover.1661240170.git.baolin.wang@linux.alibaba.com/
> Signed-off-by: Mike Kravetz <mike.kravetz@...cle.com>
[...]
> +static struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> + unsigned long address, unsigned int flags)
> +{
> + /* should never happen, but do not want to BUG */
> + return ERR_PTR(-EINVAL);
Should there be a WARN_ON_ONCE() instead or could we use a BUILD_BUG_ON()?
> +}
[...]
> @@ -851,10 +814,15 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
>
> ctx->page_mask = 0;
>
> - /* make this handle hugepd */
> - page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
> - if (!IS_ERR(page)) {
> - WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
> + /*
> + * Call hugetlb_follow_page_mask for hugetlb vmas as it will use
> + * special hugetlb page table walking code. This eliminates the
> + * need to check for hugetlb entries in the general walking code.
> + */
Maybe also comment that ordinary GUP never ends up in here and instead
directly uses follow_hugetlb_page(). This is for follow_page() handling
only.
[me suggestion to rename follow_hugetlb_page() still stands ;) ]
> + if (is_vm_hugetlb_page(vma)) {
> + page = hugetlb_follow_page_mask(vma, address, flags);
> + if (!page)
> + page = no_page_table(vma, flags);
> return page;
> }
>
> @@ -863,21 +831,6 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
> if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd)))
> return no_page_table(vma, flags);
>
> - if (pgd_huge(*pgd)) {
> - page = follow_huge_pgd(mm, address, pgd, flags);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> - if (is_hugepd(__hugepd(pgd_val(*pgd)))) {
> - page = follow_huge_pd(vma, address,
> - __hugepd(pgd_val(*pgd)), flags,
> - PGDIR_SHIFT);
> - if (page)
> - return page;
> - return no_page_table(vma, flags);
> - }
> -
> return follow_p4d_mask(vma, address, pgd, flags, ctx);
> }
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index d0617d64d718..b3da421ba5be 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6190,6 +6190,62 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte,
> return false;
> }
>
> +struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> + unsigned long address, unsigned int flags)
> +{
> + struct hstate *h = hstate_vma(vma);
> + struct mm_struct *mm = vma->vm_mm;
> + unsigned long haddr = address & huge_page_mask(h);
> + struct page *page = NULL;
> + spinlock_t *ptl;
> + pte_t *pte, entry;
> +
> + /*
> + * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> + * follow_hugetlb_page().
> + */
> + if (WARN_ON_ONCE(flags & FOLL_PIN))
> + return NULL;
> +
> + pte = huge_pte_offset(mm, haddr, huge_page_size(h));
> + if (!pte)
> + return NULL;
> +
> +retry:
> + ptl = huge_pte_lock(h, mm, pte);
> + entry = huge_ptep_get(pte);
> + if (pte_present(entry)) {
> + page = pte_page(entry) +
> + ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> + /*
> + * Note that page may be a sub-page, and with vmemmap
> + * optimizations the page struct may be read only.
> + * try_grab_page() will increase the ref count on the
> + * head page, so this will be OK.
> + *
> + * try_grab_page() should always succeed here, because we hold
> + * the ptl lock and have verified pte_present().
> + */
> + if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
> + page = NULL;
> + goto out;
> + }
> + } else {
> + if (is_hugetlb_entry_migration(entry)) {
> + spin_unlock(ptl);
> + __migration_entry_wait_huge(pte, ptl);
> + goto retry;
> + }
> + /*
> + * hwpoisoned entry is treated as no_page_table in
> + * follow_page_mask().
> + */
> + }
> +out:
> + spin_unlock(ptl);
> + return page;
> +}
> +
> long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> struct page **pages, struct vm_area_struct **vmas,
> unsigned long *position, unsigned long *nr_pages,
> @@ -7140,123 +7196,6 @@ __weak unsigned long hugetlb_mask_last_page(struct hstate *h)
> * These functions are overwritable if your architecture needs its own
> * behavior.
> */
[...]
Numbers speak for themselves.
Acked-by: David Hildenbrand <david@...hat.com>
--
Thanks,
David / dhildenb
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