linux-kernel - Re: [PATCH RESEND v2] docs: x86: Remove obsolete information about x86

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Date:   Thu, 19 Aug 2021 09:58:46 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Peilin Ye <yepeilin.cs@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Muchun Song <songmuchun@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Peilin Ye <peilin.ye@...edance.com>
Subject: Re: [PATCH RESEND v2] docs: x86: Remove obsolete information about
 x86_64 vmalloc() faulting

On Wed, Aug 18, 2021 at 03:01:23PM -0700, Peilin Ye wrote:
> From: Peilin Ye <peilin.ye@...edance.com>
> 
> x86_64 vmalloc() mappings are no longer "synchronized" among page tables
> via faulting since commit 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD
> pages for vmalloc area"), since the corresponding P4D or PUD pages are
> now preallocated at boot, by preallocate_vmalloc_pages().  Drop the
> "lazily synchronized" description for less confusion.
> 
> While this file is x86_64-specific, it is worth noting that things are
> different for x86_32, where vmalloc()-related changes to `init_mm.pgd` are
> synchronized to all page tables in the system during runtime, via
> arch_sync_kernel_mappings().  Unfortunately, this synchronization is
> subject to race condition, which is further handled via faulting, see
> vmalloc_fault().  See commit 4819e15f740e ("x86/mm/32: Bring back vmalloc
> faulting on x86_32") for more details.
> 
> Reviewed-by: Muchun Song <songmuchun@...edance.com>
> Signed-off-by: Peilin Ye <peilin.ye@...edance.com>

Reviewed-by: Joerg Roedel <jroedel@...e.de>

> ---
> Hi all,
> 
> Resending this with Muchun's Reviewed-by:.
> 
> Thanks,
> Peilin Ye
> 
> Changes in v2:
>     - More information for x86_32 in commit message (Joerg Roedel
>       <jroedel@...e.de>)
>     - Use my new email address for work
> 
>  Documentation/x86/x86_64/mm.rst | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/Documentation/x86/x86_64/mm.rst b/Documentation/x86/x86_64/mm.rst
> index ede1875719fb..9798676bb0bf 100644
> --- a/Documentation/x86/x86_64/mm.rst
> +++ b/Documentation/x86/x86_64/mm.rst
> @@ -140,10 +140,6 @@ The direct mapping covers all memory in the system up to the highest
>  memory address (this means in some cases it can also include PCI memory
>  holes).
>  
> -vmalloc space is lazily synchronized into the different PML4/PML5 pages of
> -the processes using the page fault handler, with init_top_pgt as
> -reference.
> -
>  We map EFI runtime services in the 'efi_pgd' PGD in a 64Gb large virtual
>  memory window (this size is arbitrary, it can be raised later if needed).
>  The mappings are not part of any other kernel PGD and are only available
> -- 
> 2.20.1
>