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SubjectRe: [RFC PATCH v2 2/9] mm: pgtable: Add sysctl to enable COW PTE
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On 9/27/22 09:29, Chih-En Lin wrote:
> Add a new sysctl vm.cow_pte to set MMF_COW_PTE_READY flag for enabling
> copy-on-write (COW) to the PTE page table during the next time of fork.
>
> Since it has a time gap between using the sysctl to enable the COW PTE
> and doing the fork, we use two states to determine the task that wants
> to do COW PTE or already doing it.
>
> Signed-off-by: Chih-En Lin <shiyn.lin@gmail.com>
> ---
> include/linux/pgtable.h | 6 ++++++
> kernel/fork.c | 5 +++++
> kernel/sysctl.c | 8 ++++++++
> mm/Makefile | 2 +-
> mm/cow_pte.c | 39 +++++++++++++++++++++++++++++++++++++++
> 5 files changed, 59 insertions(+), 1 deletion(-)
> create mode 100644 mm/cow_pte.c
>
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index 014ee8f0fbaab..d03d01aefe989 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -937,6 +937,12 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
> __ptep_modify_prot_commit(vma, addr, ptep, pte);
> }
> #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
> +
> +int cow_pte_handler(struct ctl_table *table, int write, void *buffer,
> + size_t *lenp, loff_t *ppos);
> +
> +extern int sysctl_cow_pte_pid;

So are setting a global value, to a single pid?? Only one pid at a time
can be set up?

I think that tells you already that there is a huge API problem here.

As the other thread with Nadav said, this is not a sysctl. It wants to
be a prctl(), at least the way things look so far.


> +
> #endif /* CONFIG_MMU */
>
> /*
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 8a9e92068b150..6981944a7c6ec 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2671,6 +2671,11 @@ pid_t kernel_clone(struct kernel_clone_args *args)
> trace = 0;
> }
>
> + if (current->mm && test_bit(MMF_COW_PTE_READY, &current->mm->flags)) {
> + clear_bit(MMF_COW_PTE_READY, &current->mm->flags);
> + set_bit(MMF_COW_PTE, &current->mm->flags);
> + }
> +
> p = copy_process(NULL, trace, NUMA_NO_NODE, args);
> add_latent_entropy();
>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 205d605cacc5b..c4f54412ae3a9 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2360,6 +2360,14 @@ static struct ctl_table vm_table[] = {
> .mode = 0644,
> .proc_handler = mmap_min_addr_handler,
> },
> + {
> + .procname = "cow_pte",
> + .data = &sysctl_cow_pte_pid,
> + .maxlen = sizeof(int),
> + .mode = 0644,
> + .proc_handler = cow_pte_handler,
> + .extra1 = SYSCTL_ZERO,
> + },
> #endif
> #ifdef CONFIG_NUMA
> {
> diff --git a/mm/Makefile b/mm/Makefile
> index 9a564f8364035..7a568d5066ee6 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -40,7 +40,7 @@ mmu-y := nommu.o
> mmu-$(CONFIG_MMU) := highmem.o memory.o mincore.o \
> mlock.o mmap.o mmu_gather.o mprotect.o mremap.o \
> msync.o page_vma_mapped.o pagewalk.o \
> - pgtable-generic.o rmap.o vmalloc.o
> + pgtable-generic.o rmap.o vmalloc.o cow_pte.o
>
>
> ifdef CONFIG_CROSS_MEMORY_ATTACH
> diff --git a/mm/cow_pte.c b/mm/cow_pte.c
> new file mode 100644
> index 0000000000000..4e50aa4294ce7
> --- /dev/null
> +++ b/mm/cow_pte.c
> @@ -0,0 +1,39 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <linux/sysctl.h>
> +#include <linux/pgtable.h>
> +#include <linux/sched.h>
> +#include <linux/sched/coredump.h>
> +#include <linux/pid.h>
> +
> +/* sysctl will write to this variable */
> +int sysctl_cow_pte_pid = -1;
> +
> +static void set_cow_pte_task(void)
> +{
> + struct pid *pid;
> + struct task_struct *task;
> +
> + pid = find_get_pid(sysctl_cow_pte_pid);

This seems to be missing a corresponding call to put_pid().

thanks,

--
John Hubbard
NVIDIA

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