Re: [PATCH] docs: x86: Remove obsolete information about x86_64 vmalloc() faulting
From: Peilin Ye <hidden>
Date: 2021-07-16 06:10:07
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Hi all,
quoted hunk ↗ jump to hunk
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.
This information is out-of-date, and it took me quite some time of
ftrace'ing before I figured it out... I think it would be beneficial to
update, or at least remove it.
As a proof that I understand what I am talking about, on my x86_64 box:
1. I allocated a vmalloc() area containing linear address `addr`;
2. I manually pagewalked `addr` in different page tables, including
`init_mm.pgd`;
3. The corresponding PGD entries for `addr` in different page tables,
they all immediately pointed at the same PUD table (my box uses
4-level paging), at the same physical address;
4. No "lazy synchronization" via page fault handling happened at all,
since it is the same PUD table pre-allocated by
preallocate_vmalloc_pages() during boot time.
Commit 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc
area") documented this clearly:
"""
Doing this at boot makes sure no synchronization of that area is
necessary at runtime.
"""
Should we remove this sentence, or update it? Any ideas?
Sincerely,
Peilin Ye