Re: [PATCH] mm: introduce ARCH_PAGE_TABLE_SYNC_MASK_VMALLOC to sync kernel mapping conditionally

[Jinliang Zheng <alexjlzheng@gmail.com>]

On Thu, 18 Sep 2025 01:41:04 +0900, harry.yoo@oracle.com wrote:
> On Wed, Sep 17, 2025 at 11:48:29PM +0800, alexjlzheng@gmail.com wrote:
> > From: Jinliang Zheng <alexjlzheng@tencent.com>
> > 
> > After commit 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for
> > vmalloc area"), we don't need to synchronize kernel mappings in the
> > vmalloc area on x86_64.
> 
> Right.
> 
> > And commit 58a18fe95e83 ("x86/mm/64: Do not sync vmalloc/ioremap
> > mappings") actually does this.
> 
> Right.
> 
> > But commit 6659d0279980 ("x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK
> > and arch_sync_kernel_mappings()") breaks this.
> 
> Good point.
> 
> > This patch introduces ARCH_PAGE_TABLE_SYNC_MASK_VMALLOC to avoid
> > unnecessary kernel mappings synchronization of the vmalloc area.
> > 
> > Fixes: 6659d0279980 ("x86/mm/64: define ARCH_PAGE_TABLE_SYNC_MASK and arch_sync_kernel_mappings()")
> 
> The commit is getting backported to -stable kernels.
> 
> Do you think this can cause a visible performance regression from
> user point of view, or it's just a nice optimization to have?
> (and any data to support?)

Haha, when I woke up in bed this morning, I suddenly realized that I
might have pushed a worthless patch and wasted everyone's precious time.

Sorry for that. :-(

After commit 6eb82f994026 ("x86/mm: Pre-allocate P4D/PUD pages for vmalloc area"),
pgd_alloc_track()/p4d_alloc_track() in vmalloc() and apply_to_range() may should
always return a mask that does not contain PGTBL_PGD_MODIFIED (5 level pgtable)
or PGTBL_P4D_MODIFIED (4 level pgtable), thereby bypassing the call to
arch_sync_kernel_mappings(). Right?

thanks,
Jinliang Zheng. :)

> 
> > Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
> > ---
> >  arch/arm/include/asm/page.h                 | 3 ++-
> >  arch/x86/include/asm/pgtable-2level_types.h | 3 ++-
> >  arch/x86/include/asm/pgtable-3level_types.h | 3 ++-
> >  include/linux/pgtable.h                     | 4 ++++
> >  mm/memory.c                                 | 2 +-
> >  mm/vmalloc.c                                | 6 +++---
> >  6 files changed, 14 insertions(+), 7 deletions(-)
> > 
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 0ba4f6b71847..cd2488043f8f 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3170,7 +3170,7 @@ static int __apply_to_page_range(struct mm_struct *mm, unsigned long addr,
> >  			break;
> >  	} while (pgd++, addr = next, addr != end);
> >  
> > -	if (mask & ARCH_PAGE_TABLE_SYNC_MASK)
> > +	if (mask & ARCH_PAGE_TABLE_SYNC_MASK_VMALLOC)
> >  		arch_sync_kernel_mappings(start, start + size);
> 
> But vmalloc is not the only user of apply_to_page_range()?
> 
> -- 
> Cheers,
> Harry / Hyeonggon