Re: [RFC PATCH 0/7] mm: Get rid of vmalloc_sync_(un)mappings()

[Andy Lutomirski]

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