linux-kernel - Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync

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Date:   Fri, 8 May 2020 14:33:19 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Joerg Roedel <jroedel@...e.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()

On Fri, May 8, 2020 at 7:40 AM Joerg Roedel <joro@...tes.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.