Message-ID: <5a06f383-7d01-4ea7-a010-013063770240@lucifer.local>
Date: Thu, 5 Jun 2025 10:24:25 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Barry Song <baohua@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Oscar Salvador <osalvador@...e.de>,
        Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
        Jonas Bonn <jonas@...thpole.se>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Stafford Horne <shorne@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
        Alexandre Ghiti <alex@...ti.fr>, Jann Horn <jannh@...gle.com>,
        loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linux-openrisc@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v2] mm/pagewalk: split walk_page_range_novma() into
 kernel/user parts

On Thu, Jun 05, 2025 at 08:56:59AM +0200, Vlastimil Babka wrote:
> On 6/4/25 16:19, Lorenzo Stoakes wrote:
> > The walk_page_range_novma() function is rather confusing - it supports two
> > modes, one used often, the other used only for debugging.
> >
> > The first mode is the common case of traversal of kernel page tables, which
> > is what nearly all callers use this for.
> >
> > Secondly it provides an unusual debugging interface that allows for the
> > traversal of page tables in a userland range of memory even for that memory
> > which is not described by a VMA.
> >
> > It is far from certain that such page tables should even exist, but perhaps
> > this is precisely why it is useful as a debugging mechanism.
> >
> > As a result, this is utilised by ptdump only. Historically, things were
> > reversed - ptdump was the only user, and other parts of the kernel evolved
> > to use the kernel page table walking here.
> >
> > Since we have some complicated and confusing locking rules for the novma
> > case, it makes sense to separate the two usages into their own functions.
> >
> > Doing this also provide self-documentation as to the intent of the caller -
> > are they doing something rather unusual or are they simply doing a standard
> > kernel page table walk?
> >
> > We therefore establish two separate functions - walk_page_range_debug() for
> > this single usage, and walk_kernel_page_table_range() for general kernel
> > page table walking.
> >
> > We additionally make walk_page_range_debug() internal to mm.
> >
> > Note that ptdump uses the precise same function for kernel walking as a
>
> IMHO it's not clear at this point what "the precise same function" means.
>
> > convenience, so we permit this but make it very explicit by having
> > walk_page_range_novma() invoke walk_kernel_page_table_range() in this case.
>
>   ^ walk_page_range_debug()

Oops will fix.

>
> Maybe this could be reworded in the sense (AFAIU) that
> walk_page_range_debug() can be used for both user space page table walking
> or kernel depending on what mm is passed, so in the case of init_mm it
> invokes walk_kernel_page_table_range() internally.

Sure.

>
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Acked-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> > ---
> > v2:
> > * Renamed walk_page_range_novma() to walk_page_range_debug() as per David.
> > * Moved walk_page_range_debug() definition to mm/internal.h as per Mike.
> > * Renamed walk_page_range_kernel() to walk_kernel_page_table_range() as
> >   per David.
> >
> > v1 resend:
> > * Actually cc'd lists...
> > * Fixed mistake in walk_page_range_novma() not handling kernel mappings and
> >   update commit message to referene.
> > * Added Mike's off-list Acked-by.
> > * Fixed up comments as per Mike.
> > * Add some historic flavour to the commit message as per Mike.
> > https://lore.kernel.org/all/20250603192213.182931-1-lorenzo.stoakes@oracle.com/
> >
> > v1:
> > (accidentally sent off-list due to error in scripting)
> >
> >  arch/loongarch/mm/pageattr.c |  2 +-
> >  arch/openrisc/kernel/dma.c   |  4 +-
> >  arch/riscv/mm/pageattr.c     |  8 +--
> >  include/linux/pagewalk.h     |  7 ++-
> >  mm/hugetlb_vmemmap.c         |  2 +-
> >  mm/internal.h                |  4 ++
> >  mm/pagewalk.c                | 98 ++++++++++++++++++++++++------------
> >  mm/ptdump.c                  |  3 +-
> >  8 files changed, 82 insertions(+), 46 deletions(-)
> >
> > diff --git a/arch/loongarch/mm/pageattr.c b/arch/loongarch/mm/pageattr.c
> > index 99165903908a..f5e910b68229 100644
> > --- a/arch/loongarch/mm/pageattr.c
> > +++ b/arch/loongarch/mm/pageattr.c
> > @@ -118,7 +118,7 @@ static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask, pgp
> >  		return 0;
> >
> >  	mmap_write_lock(&init_mm);
> > -	ret = walk_page_range_novma(&init_mm, start, end, &pageattr_ops, NULL, &masks);
> > +	ret = walk_kernel_page_table_range(start, end, &pageattr_ops, NULL, &masks);
> >  	mmap_write_unlock(&init_mm);
>
> You've removed init_mm from walk_page_range_novma() but I see most callers
> do the locking of init_mm immediately around it. This suggests a version
> handling that automatically? A bit complicated by the read/write
> possibilities, so maybe not worth wrapping? Just a thought, as David says ;)

Most callers write lock interestingly, but then one read lock's, so we can't
just assume and would need to pass a boolean which would kind of suck.

Also other walkers assume the caller has the lock so it's consistent to
keep it this way.

>
> >
> >  	flush_tlb_kernel_range(start, end);
> > diff --git a/arch/openrisc/kernel/dma.c b/arch/openrisc/kernel/dma.c
> > index 3a7b5baaa450..af932a4ad306 100644
> > --- a/arch/openrisc/kernel/dma.c
> > +++ b/arch/openrisc/kernel/dma.c
> > @@ -72,7 +72,7 @@ void *arch_dma_set_uncached(void *cpu_addr, size_t size)
> >  	 * them and setting the cache-inhibit bit.
> >  	 */
> >  	mmap_write_lock(&init_mm);
> > -	error = walk_page_range_novma(&init_mm, va, va + size,
> > +	error = walk_kernel_page_table_range(va, va + size,
> >  			&set_nocache_walk_ops, NULL, NULL);
> >  	mmap_write_unlock(&init_mm);
> >
> > @@ -87,7 +87,7 @@ void arch_dma_clear_uncached(void *cpu_addr, size_t size)
> >
> >  	mmap_write_lock(&init_mm);
> >  	/* walk_page_range shouldn't be able to fail here */
> > -	WARN_ON(walk_page_range_novma(&init_mm, va, va + size,
> > +	WARN_ON(walk_kernel_page_table_range(va, va + size,
> >  			&clear_nocache_walk_ops, NULL, NULL));
> >  	mmap_write_unlock(&init_mm);
> >  }
> > diff --git a/arch/riscv/mm/pageattr.c b/arch/riscv/mm/pageattr.c
> > index d815448758a1..3f76db3d2769 100644
> > --- a/arch/riscv/mm/pageattr.c
> > +++ b/arch/riscv/mm/pageattr.c
> > @@ -299,7 +299,7 @@ static int __set_memory(unsigned long addr, int numpages, pgprot_t set_mask,
> >  			if (ret)
> >  				goto unlock;
> >
> > -			ret = walk_page_range_novma(&init_mm, lm_start, lm_end,
> > +			ret = walk_kernel_page_table_range(lm_start, lm_end,
> >  						    &pageattr_ops, NULL, &masks);
>
> Note this and other places break the second line's arguments alignment on
> the opening bracket. Maybe it just shows it's a bit fragile style...
>
>

Yeah I know :) I know you won't believe this coming from me, but I was
trying to minimise the churn :P

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