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

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Message-Id: <20210818220123.2623-1-yepeilin.cs@gmail.com>
Date:   Wed, 18 Aug 2021 15:01:23 -0700
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     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>
Cc:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Joerg Roedel <jroedel@...e.de>,
        Muchun Song <songmuchun@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        Peilin Ye <peilin.ye@...edance.com>,
        Peilin Ye <yepeilin.cs@...il.com>
Subject: [PATCH RESEND v2] docs: x86: Remove obsolete information about x86_64 vmalloc() faulting

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>
---
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