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From:   Andy Lutomirski <luto@kernel.org>
Date:   Fri, 8 May 2020 14:33:19 -0700
Message-ID: <CALCETrX0ubjc0Gf4hCY9RWH6cVEKF1hv3RzqToKMt9_bEXXBvw@mail.gmail.com>
Subject: Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()
To:     Joerg Roedel <joro@8bytes.org>
Cc:     X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@kernel.org>,
        Joerg Roedel <jroedel@suse.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ACPI <linux-acpi@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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On Fri, May 8, 2020 at 7:40 AM Joerg Roedel <joro@8bytes.org> wrote:
>
> Hi,
>
> after the recent issue with vmalloc and tracing code[1] on x86 and a
> long history of previous issues related to the vmalloc_sync_mappings()
> interface, I thought the time has come to remove it. Please
> see [2], [3], and [4] for some other issues in the past.
>
> The patches are based on v5.7-rc4 and add tracking of page-table
> directory changes to the vmalloc and ioremap code. Depending on which
> page-table levels changes have been made, a new per-arch function is
> called: arch_sync_kernel_mappings().
>
> On x86-64 with 4-level paging, this function will not be called more
> than 64 times in a systems runtime (because vmalloc-space takes 64 PGD
> entries which are only populated, but never cleared).

What's the maximum on other system types?  It might make more sense to
take the memory hit and pre-populate all the tables at boot so we
never have to sync them.