[PATCH RESEND v2] docs: x86: Remove obsolete information about x86_64 vmalloc() faulting

From: Peilin Ye <redacted>

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 <redacted>
Signed-off-by: Peilin Ye <redacted>
---
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
      [off-list ref])
    - 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

On Wed, Aug 18, 2021 at 03:01:23PM -0700, Peilin Ye wrote:

From: Peilin Ye <redacted>

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 <redacted>
Signed-off-by: Peilin Ye <redacted>

Reviewed-by: Joerg Roedel <redacted>

Joerg Roedel [off-list ref] writes:

On Wed, Aug 18, 2021 at 03:01:23PM -0700, Peilin Ye wrote:

quoted

From: Peilin Ye <redacted>

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 <redacted>
Signed-off-by: Peilin Ye <redacted>

Reviewed-by: Joerg Roedel <redacted>

Applied, thanks.

jon