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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250918013130.2425537-1-alexjlzheng@tencent.com>
Date: Thu, 18 Sep 2025 09:31:30 +0800
From: Jinliang Zheng <alexjlzheng@...il.com>
To: harry.yoo@...cle.com
Cc: Liam.Howlett@...cle.com,
	akpm@...ux-foundation.org,
	alexjlzheng@...il.com,
	alexjlzheng@...cent.com,
	arnd@...db.de,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	david@...hat.com,
	geert@...ux-m68k.org,
	hpa@...or.com,
	joro@...tes.org,
	jroedel@...e.de,
	kas@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	linux@...linux.org.uk,
	lorenzo.stoakes@...cle.com,
	mhocko@...e.com,
	mingo@...hat.com,
	rppt@...nel.org,
	surenb@...gle.com,
	tglx@...utronix.de,
	thuth@...hat.com,
	urezki@...il.com,
	vbabka@...e.cz,
	vincenzo.frascino@....com,
	x86@...nel.org
Subject: Re: [PATCH] mm: introduce ARCH_PAGE_TABLE_SYNC_MASK_VMALLOC to sync kernel mapping conditionally

On Thu, 18 Sep 2025 01:41:04 +0900, harry.yoo@...cle.com wrote:
> On Wed, Sep 17, 2025 at 11:48:29PM +0800, alexjlzheng@...il.com wrote:
> > From: Jinliang Zheng <alexjlzheng@...cent.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@...cent.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