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From:   =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@kernel.org>
To:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org
Cc:     =?utf-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn@rivosinc.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux@rivosinc.com, Alexandre Ghiti <alexghiti@rivosinc.com>,
        Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH] riscv: mm: Pre-allocate PGD entries vmalloc/modules area
In-Reply-To: <20230529180023.289904-1-bjorn@kernel.org>
References: <20230529180023.289904-1-bjorn@kernel.org>
Date:   Tue, 30 May 2023 08:37:17 +0200
Message-ID: <871qiyfhpe.fsf@all.your.base.are.belong.to.us>
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Bj=C3=B6rn T=C3=B6pel <bjorn@kernel.org> writes:

> From: Bj=C3=B6rn T=C3=B6pel <bjorn@rivosinc.com>
>
> 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 J=C3=B6rg R=C3=B6del'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@8bytes.org=
/ # [1]
> Signed-off-by: Bj=C3=B6rn T=C3=B6pel <bjorn@rivosinc.com>

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


Bj=C3=B6rn

[1] https://patchwork.kernel.org/project/linux-riscv/patch/20230529180023.2=
89904-1-bjorn@kernel.org/