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

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

[Peilin Ye]

From: Peilin Ye <peilin.ye@bytedance.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@bytedance.com>
Signed-off-by: Peilin Ye <peilin.ye@bytedance.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@suse.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

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

[Joerg Roedel]

On Wed, Aug 18, 2021 at 03:01:23PM -0700, Peilin Ye wrote:
> From: Peilin Ye <peilin.ye@bytedance.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@bytedance.com>
> Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>

Reviewed-by: Joerg Roedel <jroedel@suse.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@suse.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
> 

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

[Jonathan Corbet]

Joerg Roedel <jroedel@suse.de> writes:

> On Wed, Aug 18, 2021 at 03:01:23PM -0700, Peilin Ye wrote:
>> From: Peilin Ye <peilin.ye@bytedance.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@bytedance.com>
>> Signed-off-by: Peilin Ye <peilin.ye@bytedance.com>
>
> Reviewed-by: Joerg Roedel <jroedel@suse.de>

Applied, thanks.

jon