linux-kernel - Re: [PATCH v12 0/4] fix double page fault in cow_user

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Message-ID: <20191015001834.wwkd46t6dwicta7n@willie-the-truck>
Date:   Tue, 15 Oct 2019 01:18:34 +0100
From:   Will Deacon <will@...nel.org>
To:     Jia He <justin.he@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        James Morse <james.morse@....com>,
        Marc Zyngier <maz@...nel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Suzuki Poulose <Suzuki.Poulose@....com>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>, hejianet@...il.com,
        Kaly Xin <Kaly.Xin@....com>, nd@....com
Subject: Re: [PATCH v12 0/4] fix double page fault in cow_user_page for pfn
 mapping

On Fri, Oct 11, 2019 at 10:09:35PM +0800, Jia He wrote:
> When we tested pmdk unit test vmmalloc_fork TEST1 in arm64 guest, there
> will be a double page fault in __copy_from_user_inatomic of cow_user_page.
> 
> As told by Catalin: "On arm64 without hardware Access Flag, copying from
> user will fail because the pte is old and cannot be marked young. So we
> always end up with zeroed page after fork() + CoW for pfn mappings. we
> don't always have a hardware-managed access flag on arm64."
> 
> -Changes
> v12:
>     refine PATCH 01, remove the !! since C languages can convert unsigned
>     to bool (Catalin)

Thanks. I think it's a bit late to take something like this for 5.4 now,
especially as the current behaviour has always been there. Hopefully
somebody can queue it for 5.5 instead.

Will