linux-kernel - Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE

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Date:   Fri, 22 Apr 2022 20:22:32 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        "npiggin@...il.com" <npiggin@...il.com>
CC:     "songliubraving@...com" <songliubraving@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "hch@...radead.org" <hch@...radead.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "Kernel-team@...com" <Kernel-team@...com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "song@...nel.org" <song@...nel.org>,
        "pmladek@...e.com" <pmladek@...e.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "mcgrof@...nel.org" <mcgrof@...nel.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "imbrenda@...ux.ibm.com" <imbrenda@...ux.ibm.com>
Subject: Re: [PATCH v4 bpf 0/4] vmalloc: bpf: introduce VM_ALLOW_HUGE_VMAP

On Fri, 2022-04-22 at 10:10 -0700, Edgecombe, Richard P wrote:
> The risk this entails is racing against all the software page table
> walks and walking a freed table. At least on x86 the walks of the
> kernel tables are done with no locks, which works because the PTE
> updates are atomic and pretty much never freed. Some of the kernel
> page
> table walks in the fault handler are actually triggerable from
> userspace.

Argh, please ignore this. I guess interrupts getting disabled in the
fault handler forces the freeing of the page table to synchronize with
the fault handler walks via the TLB flush IPIs that precede the free.
Sorry for the noise.