Re: [PATCH v4 1/3] mm: add new api to enable ksm per process

On 10.03.23 19:28, Stefan Roesch wrote:

Patch series "mm: process/cgroup ksm support", v3.

So far KSM can only be enabled by calling madvise for memory regions. To be able to use KSM for more workloads, KSM needs to have the ability to be enabled / disabled at the process / cgroup level.

Use case 1:

The madvise call is not available in the programming language. An example for this are programs with forked workloads using a garbage collected language without pointers. In such a language madvise cannot be made available.

In addition the addresses of objects get moved around as they are garbage collected. KSM sharing needs to be enabled "from the outside" for these type of workloads.

I guess the interpreter could enable it (like a memory allocator could enable it for the whole heap). But I get that it's much easier to enable this per-process, and eventually only when a lot of the same processes are running in that particular environment.

Use case 2:

The same interpreter can also be used for workloads where KSM brings no benefit or even has overhead. We'd like to be able to enable KSM on a workload by workload basis.

Agreed. A per-process control is also helpful to identidy workloads where KSM might be beneficial (and to which degree).

Use case 3:

With the madvise call sharing opportunities are only enabled for the current process: it is a workload-local decision. A considerable number of sharing opportuniites may exist across multiple workloads or jobs. Only a higler level entity like a job scheduler or container can know for certain if its running one or more instances of a job. That job scheduler however doesn't have the necessary internal worklaod knowledge to make targeted madvise calls.

Security concerns:

In previous discussions security concerns have been brought up. The problem is that an individual workload does not have the knowledge about what else is running on a machine. Therefore it has to be very conservative in what memory areas can be shared or not. However, if the system is dedicated to running multiple jobs within the same security domain, its the job scheduler that has the knowledge that sharing can be safely enabled and is even desirable.

Performance:

Experiments with using UKSM have shown a capacity increase of around 20%.

As raised, it would be great to include more details about the workload where this particulalry helps (e.g., a lot of Django processes operating in the same domain).

1. New options for prctl system command

   This patch series adds two new options to the prctl system call. The first one allows to enable KSM at the process level and the second one to query the setting.

   The setting will be inherited by child processes.

   With the above setting, KSM can be enabled for the seed process of a cgroup and all processes in the cgroup will inherit the setting.

2. Changes to KSM processing

   When KSM is enabled at the process level, the KSM code will iterate over all the VMA's and enable KSM for the eligible VMA's.

   When forking a process that has KSM enabled, the setting will be inherited by the new child process.

   In addition when KSM is disabled for a process, KSM will be disabled for the VMA's where KSM has been enabled.

Do we want to make MADV_MERGEABLE/MADV_UNMERGEABLE fail while the new prctl is enabled for a process?

3. Add general_profit metric

   The general_profit metric of KSM is specified in the documentation, but not calculated. This adds the general profit metric to /sys/kernel/debug/mm/ksm.

4. Add more metrics to ksm_stat

   This adds the process profit and ksm type metric to /proc/<pid>/ksm_stat.

5. Add more tests to ksm_tests

   This adds an option to specify the merge type to the ksm_tests. This allows to test madvise and prctl KSM. It also adds a new option to query if prctl KSM has been enabled. It adds a fork test to verify that the KSM process setting is inherited by client processes.

An update to the prctl(2) manpage has been proposed at [1].

This patch (of 3):

This adds a new prctl to API to enable and disable KSM on a per process basis instead of only at the VMA basis (with madvise).

1. Introduce new MMF_VM_MERGE_ANY flag

   This introduces the new flag MMF_VM_MERGE_ANY flag. When this flag is set, kernel samepage merging (ksm) gets enabled for all vma's of a process.

2. add flag to __ksm_enter

   This change adds the flag parameter to __ksm_enter. This allows to distinguish if ksm was called by prctl or madvise.

3. add flag to __ksm_exit call

   This adds the flag parameter to the __ksm_exit() call. This allows to distinguish if this call is for an prctl or madvise invocation.

4. invoke madvise for all vmas in scan_get_next_rmap_item

   If the new flag MMF_VM_MERGE_ANY has been set for a process, iterate over all the vmas and enable ksm if possible. For the vmas that can be ksm enabled this is only done once.

5. support disabling of ksm for a process

   This adds the ability to disable ksm for a process if ksm has been enabled for the process.

6. add new prctl option to get and set ksm for a process

   This adds two new options to the prctl system call

   • enable ksm for all vmas of a process (if the vmas support it).
   • query if ksm has been enabled for a process.

Did you consider, instead of handling MMF_VM_MERGE_ANY in a special way, to instead make it reuse the existing MMF_VM_MERGEABLE/VM_MERGEABLE infrastructure. Especially:

1) During prctl(MMF_VM_MERGE_ANY), set VM_MERGABLE on all applicable compatible. Further, set MMF_VM_MERGEABLE and enter KSM if not already set.

2) When creating a new, compatible VMA and MMF_VM_MERGE_ANY is set, set VM_MERGABLE?

The you can avoid all runtime checks for compatible VMAs and only look at the VM_MERGEABLE flag. In fact, the VM_MERGEABLE will be completely expressive then for all VMAs. You don't need vma_ksm_mergeable() then.

Another thing to consider is interaction with arch/s390/mm/gmap.c: s390x/kvm does not support KSM and it has to disable it for all VMAs. We have to find a way to fence the prctl (for example, fail setting the prctl after gmap_mark_unmergeable() ran, and make gmap_mark_unmergeable() fail if the prctl ran -- or handle it gracefully in some other way).

Link: https://lkml.kernel.org/r/20230227220206.436662-1-shr@devkernel.io [1] Link: https://lkml.kernel.org/r/20230224044000.3084046-1-shr@devkernel.io Link: https://lkml.kernel.org/r/20230224044000.3084046-2-shr@devkernel.io Signed-off-by: Stefan Roesch shr@devkernel.io Cc: David Hildenbrand david@redhat.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Michal Hocko mhocko@suse.com Cc: Rik van Riel riel@surriel.com Cc: Bagas Sanjaya bagasdotme@gmail.com Signed-off-by: Andrew Morton akpm@linux-foundation.org

---

include/linux/ksm.h | 14 ++++-- include/linux/sched/coredump.h | 1 + include/uapi/linux/prctl.h | 2 + kernel/sys.c | 27 ++++++++++ mm/ksm.c | 90 +++++++++++++++++++++++----------- 5 files changed, 101 insertions(+), 33 deletions(-)

diff --git a/include/linux/ksm.h b/include/linux/ksm.h index 7e232ba59b86..d38a05a36298 100644 --- a/include/linux/ksm.h +++ b/include/linux/ksm.h @@ -18,20 +18,24 @@ #ifdef CONFIG_KSM int ksm_madvise(struct vm_area_struct *vma, unsigned long start, unsigned long end, int advice, unsigned long *vm_flags); -int __ksm_enter(struct mm_struct *mm); -void __ksm_exit(struct mm_struct *mm); +int __ksm_enter(struct mm_struct *mm, int flag); +void __ksm_exit(struct mm_struct *mm, int flag); static inline int ksm_fork(struct mm_struct *mm, struct mm_struct *oldmm) {

• if (test_bit(MMF_VM_MERGE_ANY, &oldmm->flags))

• return __ksm_enter(mm, MMF_VM_MERGE_ANY);
  

  if (test_bit(MMF_VM_MERGEABLE, &oldmm->flags))

• return __ksm_enter(mm);
  

• return __ksm_enter(mm, MMF_VM_MERGEABLE);
  

  return 0; }

static inline void ksm_exit(struct mm_struct *mm) {

• if (test_bit(MMF_VM_MERGEABLE, &mm->flags))

• __ksm_exit(mm);
  

• if (test_bit(MMF_VM_MERGE_ANY, &mm->flags))

• __ksm_exit(mm, MMF_VM_MERGE_ANY);
  

• else if (test_bit(MMF_VM_MERGEABLE, &mm->flags))

• __ksm_exit(mm, MMF_VM_MERGEABLE);
  

  }

/* diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h index 0e17ae7fbfd3..0ee96ea7a0e9 100644 --- a/include/linux/sched/coredump.h +++ b/include/linux/sched/coredump.h @@ -90,4 +90,5 @@ static inline int get_dumpable(struct mm_struct *mm) #define MMF_INIT_MASK (MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\ MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK) +#define MMF_VM_MERGE_ANY 29 #endif /* _LINUX_SCHED_COREDUMP_H */ diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 1312a137f7fb..759b3f53e53f 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -290,4 +290,6 @@ struct prctl_mm_map { #define PR_SET_VMA 0x53564d41 # define PR_SET_VMA_ANON_NAME 0 +#define PR_SET_MEMORY_MERGE 67 +#define PR_GET_MEMORY_MERGE 68 #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/sys.c b/kernel/sys.c index 495cd87d9bf4..edc439b1cae9 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -15,6 +15,7 @@ #include <linux/highuid.h> #include <linux/fs.h> #include <linux/kmod.h> +#include <linux/ksm.h> #include <linux/perf_event.h> #include <linux/resource.h> #include <linux/kernel.h> @@ -2661,6 +2662,32 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, case PR_SET_VMA: error = prctl_set_vma(arg2, arg3, arg4, arg5); break; +#ifdef CONFIG_KSM

• case PR_SET_MEMORY_MERGE:

• if (!capable(CAP_SYS_RESOURCE))
  

• 	return -EPERM;
  

• if (arg2) {
  

• 	if (mmap_write_lock_killable(me->mm))
  

• 		return -EINTR;
  

• 	if (!test_bit(MMF_VM_MERGE_ANY, &me->mm->flags))
  

• 		error = __ksm_enter(me->mm, MMF_VM_MERGE_ANY);
  

Hm, I think this might be problematic if we alread called __ksm_enter() via madvise(). Maybe we should really consider making MMF_VM_MERGE_ANY set MMF_VM_MERGABLE instead. Like:

error = 0; if(test_bit(MMF_VM_MERGEABLE, &me->mm->flags)) error = __ksm_enter(me->mm); if (!error) set_bit(MMF_VM_MERGE_ANY, &me->mm->flags);

• 	mmap_write_unlock(me->mm);
  

• } else {
  

• 	__ksm_exit(me->mm, MMF_VM_MERGE_ANY);
  

Hm, I'd prefer if we really only call __ksm_exit() when we really exit the process. Is there a strong requirement to optimize disabling of KSM or would it be sufficient to clear the MMF_VM_MERGE_ANY flag here?

Also, I wonder what happens if we have another VMA in that process that has it enabled ..

Last but not least, wouldn't we want to do the same thing as MADV_UNMERGEABLE and actually unmerge the KSM pages?

It smells like it could be simpler and more consistent to handle by letting PR_SET_MEMORY_MERGE piggy-back on MMF_VM_MERGABLE/VM_MERGABLE and mimic what ksm_madvise() does simply for all VMAs.

--- a/mm/ksm.c +++ b/mm/ksm.c @@ -534,16 +534,58 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr, return (ret & VM_FAULT_OOM) ? -ENOMEM : 0; } +static bool vma_ksm_compatible(struct vm_area_struct *vma) +{

• /*

• * Be somewhat over-protective for now!
  

• */
  

• if (vma->vm_flags & (VM_MERGEABLE | VM_SHARED | VM_MAYSHARE |

• 	     VM_PFNMAP    | VM_IO      | VM_DONTEXPAND |
  

• 	     VM_HUGETLB | VM_MIXEDMAP))
  

• return false;		/* just ignore the advice */
  

That comment is kind-of stale and ksm_madvise() specific.

The VM_MERGEABLE check is really only used for ksm_madvise() to return immediately. I suggest keeping it in ksm_madvise() -- "Already enabled". Returning "false" in that case looks wrong (it's not broken because you do an early check in vma_ksm_mergeable(), it's just semantically weird).