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Message-ID: <ZJsV19f41CrfkFYa@ziepe.ca>
Date: Tue, 27 Jun 2023 14:01:11 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Hugh Dickins <hughd@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Mike Rapoport <rppt@...nel.org>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Matthew Wilcox <willy@...radead.org>,
David Hildenbrand <david@...hat.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Qi Zheng <zhengqi.arch@...edance.com>,
Yang Shi <shy828301@...il.com>,
Mel Gorman <mgorman@...hsingularity.net>,
Peter Xu <peterx@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
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Ralph Campbell <rcampbell@...dia.com>,
Ira Weiny <ira.weiny@...el.com>,
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SeongJae Park <sj@...nel.org>,
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Vlastimil Babka <vbabka@...e.cz>,
linux-arm-kernel@...ts.infradead.org, sparclinux@...r.kernel.org,
linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 05/12] powerpc: add pte_free_defer() for pgtables
sharing page
On Wed, Jun 21, 2023 at 07:36:11PM -0700, Hugh Dickins wrote:
> [PATCH v3 05/12] powerpc: add pte_free_defer() for pgtables sharing page
>
> Add powerpc-specific pte_free_defer(), to free table page via call_rcu().
> pte_free_defer() will be called inside khugepaged's retract_page_tables()
> loop, where allocating extra memory cannot be relied upon. This precedes
> the generic version to avoid build breakage from incompatible pgtable_t.
>
> This is awkward because the struct page contains only one rcu_head, but
> that page may be shared between PTE_FRAG_NR pagetables, each wanting to
> use the rcu_head at the same time. But powerpc never reuses a fragment
> once it has been freed: so mark the page Active in pte_free_defer(),
> before calling pte_fragment_free() directly; and there call_rcu() to
> pte_free_now() when last fragment is freed and the page is PageActive.
>
> Suggested-by: Jason Gunthorpe <jgg@...pe.ca>
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
> arch/powerpc/include/asm/pgalloc.h | 4 ++++
> arch/powerpc/mm/pgtable-frag.c | 29 ++++++++++++++++++++++++++---
> 2 files changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/pgalloc.h b/arch/powerpc/include/asm/pgalloc.h
> index 3360cad78ace..3a971e2a8c73 100644
> --- a/arch/powerpc/include/asm/pgalloc.h
> +++ b/arch/powerpc/include/asm/pgalloc.h
> @@ -45,6 +45,10 @@ static inline void pte_free(struct mm_struct *mm, pgtable_t ptepage)
> pte_fragment_free((unsigned long *)ptepage, 0);
> }
>
> +/* arch use pte_free_defer() implementation in arch/powerpc/mm/pgtable-frag.c */
> +#define pte_free_defer pte_free_defer
> +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable);
> +
> /*
> * Functions that deal with pagetables that could be at any level of
> * the table need to be passed an "index_size" so they know how to
> diff --git a/arch/powerpc/mm/pgtable-frag.c b/arch/powerpc/mm/pgtable-frag.c
> index 20652daa1d7e..0c6b68130025 100644
> --- a/arch/powerpc/mm/pgtable-frag.c
> +++ b/arch/powerpc/mm/pgtable-frag.c
> @@ -106,6 +106,15 @@ pte_t *pte_fragment_alloc(struct mm_struct *mm, int kernel)
> return __alloc_for_ptecache(mm, kernel);
> }
>
> +static void pte_free_now(struct rcu_head *head)
> +{
> + struct page *page;
> +
> + page = container_of(head, struct page, rcu_head);
> + pgtable_pte_page_dtor(page);
> + __free_page(page);
> +}
> +
> void pte_fragment_free(unsigned long *table, int kernel)
> {
> struct page *page = virt_to_page(table);
> @@ -115,8 +124,22 @@ void pte_fragment_free(unsigned long *table, int kernel)
>
> BUG_ON(atomic_read(&page->pt_frag_refcount) <= 0);
> if (atomic_dec_and_test(&page->pt_frag_refcount)) {
> - if (!kernel)
> - pgtable_pte_page_dtor(page);
> - __free_page(page);
> + if (kernel)
> + __free_page(page);
> + else if (TestClearPageActive(page))
> + call_rcu(&page->rcu_head, pte_free_now);
> + else
> + pte_free_now(&page->rcu_head);
> }
> }
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +void pte_free_defer(struct mm_struct *mm, pgtable_t pgtable)
> +{
> + struct page *page;
> +
> + page = virt_to_page(pgtable);
> + SetPageActive(page);
> + pte_fragment_free((unsigned long *)pgtable, 0);
> +}
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
Yes, this makes sense to me, very simple..
I always for get these details but atomic_dec_and_test() is a release?
So the SetPageActive is guarenteed to be visible in another thread
that reaches 0?
Thanks,
Jason
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