linux-kernel - Re: [PATCH v3 07/13] s390: add pte_free

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Message-ID: <20230719162506.235856eb@p-imbrenda>
Date:   Wed, 19 Jul 2023 16:25:06 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
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        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 v3 07/13] s390: add pte_free_defer() for pgtables
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On Tue, 11 Jul 2023 21:38:35 -0700 (PDT)
Hugh Dickins <hughd@...gle.com> wrote:

[...]

> +#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);
> +	page_table_free(mm, (unsigned long *)pgtable);
> +	/*
> +	 * page_table_free() does not do the pgste gmap_unlink() which
> +	 * page_table_free_rcu() does: warn us if pgste ever reaches here.
> +	 */
> +	WARN_ON_ONCE(mm_alloc_pgste(mm));

it seems I have overlooked something when we previously discussed
this...

mm_alloc_pgste() is true for all processes that have PGSTEs, not only
for processes that can run guests.

There are two ways to enable PGSTEs: an ELF header bit, and a sysctl
knob.

The ELF bit is only used by qemu, it enables PGSTE allocation only for
that single process. This is a strong indication that the process wants
to run guests.

The sysctl knob enables PGSTE allocation for every process in the system
from that moment on. In that case, the WARN_ON_ONCE would be triggered
when not necessary.

There is however another way to check if a process is actually
__using__ the PGSTEs, a.k.a. if the process is actually capable of
running guests.

Confusingly, the name of that function is mm_has_pgste(). This confused
me as well, which is why I didn't notice it when we discussed this
previously :)


in short: can you please use mm_has_pgste() instead of mm_alloc_pgste()
in the WARN_ON_ONCE ?

> +}
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
>  /*
>   * Base infrastructure required to generate basic asces, region, segment,
>   * and page tables that do not make use of enhanced features like EDAT1.