[PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[Suren Baghdasaryan]

vma_copy() function for !CONFIG_PER_VMA_LOCK configuration copies all
fields using memcpy() as opposed to CONFIG_PER_VMA_LOCK version which
copies only required fields. anon_vma_chain field should not be copied
and new vma should instead initialize it to an empty list. Fix this
by initializing anon_vma_chain inside vma_copy() function. The version
of vma_copy() for CONFIG_PER_VMA_LOCK is fine since it does not change
that field and anon_vma_chain of any new vma is already initialized and
empty.

Fixes: 85ad413389ae ("mm: make vma cache SLAB_TYPESAFE_BY_RCU")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202412082208.db1fb2c9-lkp@intel.com
Reported-by: Klara Modin <klarasmodin@gmail.com>
Closes: https://lore.kernel.org/all/d0ae7609-aca4-4497-9188-bb09e96e7768@gmail.com/
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
Applies over mm-unstable

 kernel/fork.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index fec32aa06135..d532f893e977 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -524,6 +524,7 @@ static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *de
 	 * will be reinitialized.
 	 */
 	data_race(memcpy(dest, src, sizeof(*dest)));
+	INIT_LIST_HEAD(&dest->anon_vma_chain);
 }
 
 #endif /* CONFIG_PER_VMA_LOCK */

base-commit: 6e165f54437931f329d09dca6c19d99af08a36e1
-- 
2.47.0.338.g60cca15819-goog

Re: [PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[Klara Modin]

On 2024-12-09 23:10, Suren Baghdasaryan wrote:
> vma_copy() function for !CONFIG_PER_VMA_LOCK configuration copies all
> fields using memcpy() as opposed to CONFIG_PER_VMA_LOCK version which
> copies only required fields. anon_vma_chain field should not be copied
> and new vma should instead initialize it to an empty list. Fix this
> by initializing anon_vma_chain inside vma_copy() function. The version
> of vma_copy() for CONFIG_PER_VMA_LOCK is fine since it does not change
> that field and anon_vma_chain of any new vma is already initialized and
> empty.
> 
> Fixes: 85ad413389ae ("mm: make vma cache SLAB_TYPESAFE_BY_RCU")
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202412082208.db1fb2c9-lkp@intel.com
> Reported-by: Klara Modin <klarasmodin@gmail.com>
> Closes: https://lore.kernel.org/all/d0ae7609-aca4-4497-9188-bb09e96e7768@gmail.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> Applies over mm-unstable
> 
>   kernel/fork.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index fec32aa06135..d532f893e977 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -524,6 +524,7 @@ static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *de
>   	 * will be reinitialized.
>   	 */
>   	data_race(memcpy(dest, src, sizeof(*dest)));
> +	INIT_LIST_HEAD(&dest->anon_vma_chain);
>   }
>   
>   #endif /* CONFIG_PER_VMA_LOCK */
> 
> base-commit: 6e165f54437931f329d09dca6c19d99af08a36e1

This fixes the issue for me.

Thanks,
Tested-by: Klara Modin <klarasmodin@gmail.com>

Re: [PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[David Hildenbrand]

On 09.12.24 23:10, Suren Baghdasaryan wrote:
> vma_copy() function for !CONFIG_PER_VMA_LOCK configuration copies all
> fields using memcpy() as opposed to CONFIG_PER_VMA_LOCK version which
> copies only required fields. anon_vma_chain field should not be copied
> and new vma should instead initialize it to an empty list. Fix this
> by initializing anon_vma_chain inside vma_copy() function. The version
> of vma_copy() for CONFIG_PER_VMA_LOCK is fine since it does not change
> that field and anon_vma_chain of any new vma is already initialized and
> empty.

I'm wondering if there is sufficient reason to have two implementations 
to do the copying.

How expensive would it be to simply use the CONFIG_PER_VMA_LOCK variant 
unconditionally? Is it even measurable in fork() micro-benchmarks?

-- 
Cheers,

David / dhildenb

Re: [PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[Lorenzo Stoakes]

On Mon, Dec 09, 2024 at 02:10:28PM -0800, Suren Baghdasaryan wrote:
> vma_copy() function for !CONFIG_PER_VMA_LOCK configuration copies all
> fields using memcpy() as opposed to CONFIG_PER_VMA_LOCK version which
> copies only required fields. anon_vma_chain field should not be copied
> and new vma should instead initialize it to an empty list. Fix this
> by initializing anon_vma_chain inside vma_copy() function. The version
> of vma_copy() for CONFIG_PER_VMA_LOCK is fine since it does not change
> that field and anon_vma_chain of any new vma is already initialized and
> empty.

Yeah ouch, good spot though.

This anon_vma stuff is landmine after landmine...

>
> Fixes: 85ad413389ae ("mm: make vma cache SLAB_TYPESAFE_BY_RCU")
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Closes: https://lore.kernel.org/oe-lkp/202412082208.db1fb2c9-lkp@intel.com
> Reported-by: Klara Modin <klarasmodin@gmail.com>
> Closes: https://lore.kernel.org/all/d0ae7609-aca4-4497-9188-bb09e96e7768@gmail.com/
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

> ---
> Applies over mm-unstable
>
>  kernel/fork.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index fec32aa06135..d532f893e977 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c

Unrelated to patch but it _really_ truly sucks to have this in
kernel/fork.c. Maybe unavoidable given we put a bunch of this kind of logic
there, but would be good to change this at some point...

> @@ -524,6 +524,7 @@ static void vma_copy(const struct vm_area_struct *src, struct vm_area_struct *de
>  	 * will be reinitialized.
>  	 */
>  	data_race(memcpy(dest, src, sizeof(*dest)));
> +	INIT_LIST_HEAD(&dest->anon_vma_chain);

Nit, but might be worth a comment here. Though I suppose it's kind of implicit.

>  }
>
>  #endif /* CONFIG_PER_VMA_LOCK */
>
> base-commit: 6e165f54437931f329d09dca6c19d99af08a36e1
> --
> 2.47.0.338.g60cca15819-goog
>

Re: [PATCH 1/1] mm: fix vma_copy for !CONFIG_PER_VMA_LOCK

[Suren Baghdasaryan]

On Tue, Dec 10, 2024 at 1:06 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 09.12.24 23:10, Suren Baghdasaryan wrote:
> > vma_copy() function for !CONFIG_PER_VMA_LOCK configuration copies all
> > fields using memcpy() as opposed to CONFIG_PER_VMA_LOCK version which
> > copies only required fields. anon_vma_chain field should not be copied
> > and new vma should instead initialize it to an empty list. Fix this
> > by initializing anon_vma_chain inside vma_copy() function. The version
> > of vma_copy() for CONFIG_PER_VMA_LOCK is fine since it does not change
> > that field and anon_vma_chain of any new vma is already initialized and
> > empty.
>
> I'm wondering if there is sufficient reason to have two implementations
> to do the copying.
>
> How expensive would it be to simply use the CONFIG_PER_VMA_LOCK variant
> unconditionally? Is it even measurable in fork() micro-benchmarks?

Yeah, the benefit if any would be tiny. I'll try measuring the
difference and if it's not visible will remove the
!CONFIG_PER_VMA_LOCK version.
Thanks!

>
> --
> Cheers,
>
> David / dhildenb
>