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

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Message-Id: <20210622031910.141262-1-yepeilin.cs@gmail.com>
Date:   Mon, 21 Jun 2021 23:19:10 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     x86@...nel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Jonathan Corbet <corbet@....net>
Cc:     "H. Peter Anvin" <hpa@...or.com>, Joerg Roedel <jroedel@...e.de>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Zefang Han <hanzefang@...il.com>,
        Wei Lin Chang <r09922117@...e.ntu.edu.tw>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Peilin Ye <yepeilin.cs@...il.com>
Subject: [PATCH] docs: x86: Remove obsolete information about x86_64 vmalloc() faulting

x86_64 vmalloc() mappings are no longer "lazily synchronized" among page
tables via page fault handling since commit 7f0a002b5a21 ("x86/mm: remove
vmalloc faulting").  Subsequently, commit 6eb82f994026 ("x86/mm:
Pre-allocate P4D/PUD pages for vmalloc area") rendered it unnecessary to
synchronize, whether lazily or not, x86_64 vmalloc() mappings at runtime,
since the corresponding P4D or PUD pages are now preallocated during
system initialization by preallocate_vmalloc_pages().  Drop the "lazily
synchronized" description for less confusion.

It is worth noting, however, that there is still a slight complication for
x86_32; see commit 4819e15f740e ("x86/mm/32: Bring back vmalloc faulting
on x86_32") for details.

Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>
---
Hi all,

I was trying to understand vmalloc() when I saw this "lazily synchronized"
statement, which confused me for a while.  Please correct me if my
understanding is wrong or out of date.

Thank you,
Peilin Ye

 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
-- 
2.25.1