Re: [PATCH] riscv: mm: Pre-allocate PGD entries vmalloc/modules area

[Björn Töpel]

Björn Töpel <bjorn@xxxxxxxxxx> writes:

> From: Björn Töpel <bjorn@xxxxxxxxxxxx>
>
> The RISC-V port requires that kernel PGD entries are to be
> synchronized between MMs. This is done via the vmalloc_fault()
> function, that simply copies the PGD entries from init_mm to the
> faulting one.
>
> Historically, faulting in PGD entries have been a source for both bugs
> [1], and poor performance.
>
> One way to get rid of vmalloc faults is by pre-allocating the PGD
> entries. Pre-allocating the entries potientially wastes 64 * 4K (65 on
> SV39). The pre-allocation function is pulled from Jörg Rödel's x86
> work, with the addition of 3-level page tables (PMD allocations).
>
> The pmd_alloc() function needs the ptlock cache to be initialized
> (when split page locks is enabled), so the pre-allocation is done in a
> RISC-V specific pgtable_cache_init() implementation.
>
> Pre-allocate the kernel PGD entries for the vmalloc/modules area, but
> only for 64b platforms.
>
> Link: https://lore.kernel.org/lkml/20200508144043.13893-1-joro@xxxxxxxxxx/ # [1]
> Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxxxx>

...and the build was bitten by nommu [1]. I'll hold off the v2 a bit, to
see if there's any other comments.


Björn

[1] https://patchwork.kernel.org/project/linux-riscv/patch/20230529180023.289904-1-bjorn@xxxxxxxxxx/