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

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Date:   Thu, 21 Apr 2022 15:47:21 +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>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "ast@...nel.org" <ast@...nel.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>,
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        "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 Thu, 2022-04-21 at 18:57 +1000, Nicholas Piggin wrote:
> Those were (AFAIKS) all in arch code though. The patch was the
> fundamental issue for x86 because it had bugs.

This wasn't root caused to arch code:
"BUG: Bad page state in process systemd-udevd"

https://lore.kernel.org/all/14444103-d51b-0fb3-ee63-c3f182f0b546@molgen.mpg.de/

In fact it wasn't root caused at all. But on the surface it seemed like
it didn't have to do with virtual page size assumptions. I wonder if it
might have to do with the vmalloc huge pages using compound pages, then
some caller doing vmalloc_to_page() and getting surprised with what
they could get away with in the struct page. But, regardless there was
an assumption, not proven, that there was some lurking cross-arch issue
that could show up for any vmalloc huge page user.

There is another good reason to opt-in to the current vmalloc huge page
size implementation - vmalloc will round up to huge page size once the
size exceeds the huge page size. Only the callers can know if the
allocation is worth using extra memory for.