On Sat, Feb 19, 2011 at 7:54 PM, Hugh Dickins <hu...@google.com> wrote:
> On Sat, 19 Feb 2011, supercilious.d...@gmail.com wrote:
>>
>> Is there a way to enable KSM globally for all eligible pages in the system
>> such that applications need not call madvise() themselves? If not, is there
>> a way to do so on behalf of a particular application without an ugly
>> LD_PRELOAD hack?
>
> Sorry, there is not.
>
>>
>> Perhaps there is a small kernel modification that might make this possible
>> or provide a sysfs flag that enables it (off by default of course)?
>
> Indeed it could be added if there were a general call for it; but ksmd
> would tend to get wasteful, and the only call for it that I remember is
> for testing.
>
> I expect you've noticed the transparent_hugepage/enabled "always" option,
> and you are thinking something like that could be done for KSM: yes.
>
>>
>> I read through the code very briefly, and as far as I can tell, I need to
>> add the VM_MERGEABLE flag to all eligible VMAs at the time they are created
>> and also __ksm_enter() the associated mm_struct at creation also. Would that
>> work?
>
> I think that's right.
>
> Here is the hack I use myself for testing KSM: just boot with option
> "allksm" (which here disables randomize_va_space as a side-effect,
> to widen the scope of sharing: cut that line out if you prefer).
It would be nice to have such a "allksm" in cgroups' memory resource controller.
Especially LXC could benefit from it.
What do you think?
> You may be puzzled by the squashed comments etc: just trying not to
> interfere with the line numbering in mm/mmap.c, in case I have other
> patches to apply there, or hit a BUG to report.
>
> Hugh
> ---
>
> mm/mmap.c | 27 +++++++++++++++++----------
> 1 file changed, 17 insertions(+), 10 deletions(-)
>
> --- 2.6.37/mm/mmap.c 2011-01-04 16:50:19.000000000 -0800
> +++ allksm/mm/mmap.c 2011-01-04 19:47:16.000000000 -0800
> @@ -960,9 +960,9 @@ void vm_stat_account(struct mm_struct *m
> #endif /* CONFIG_PROC_FS */
>
> /*
> - * The caller must hold down_write(¤t->mm->mmap_sem).
> - */
> -
> + * The caller must hold down_write(¤t->mm->mmap_sem). */
> +#include <linux/ksm.h>
> +unsigned long vm_mergeable;
> unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
> unsigned long len, unsigned long prot,
> unsigned long flags, unsigned long pgoff)
> @@ -1086,7 +1086,7 @@ unsigned long do_mmap_pgoff(struct file
> /*
> * Set pgoff according to addr for anon_vma.
> */
> - pgoff = addr >> PAGE_SHIFT;
> + vm_flags |= vm_mergeable; pgoff = addr >> PAGE_SHIFT;
> break;
> default:
> return -EINVAL;
> @@ -1303,10 +1303,10 @@ munmap_back:
> vma->vm_file = file;
> get_file(file);
> error = file->f_op->mmap(file, vma);
> - if (error)
> - goto unmap_and_free_vma;
> - if (vm_flags & VM_EXECUTABLE)
> - added_exe_file_vma(mm);
> + if (error) goto unmap_and_free_vma;
> + if (vm_flags & VM_EXECUTABLE) added_exe_file_vma(mm);
> + if (vm_mergeable)
> + ksm_madvise(vma, 0, 0, MADV_MERGEABLE,&vma->vm_flags);
>
> /* Can addr have changed??
> *
> @@ -2167,7 +2167,7 @@ unsigned long do_brk(unsigned long addr,
> return error;
>
> flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
> -
> + flags |= vm_mergeable;
> error = get_unmapped_area(NULL, addr, len, 0, MAP_FIXED);
> if (error & ~PAGE_MASK)
> return error;
> @@ -2318,7 +2318,7 @@ int insert_vm_struct(struct mm_struct *
> if (!vma->vm_file) {
> BUG_ON(vma->anon_vma);
> vma->vm_pgoff = vma->vm_start >> PAGE_SHIFT;
> - }
> + vma->vm_flags |= vm_mergeable; }
> __vma = find_vma_prepare(mm,vma->vm_start,&prev,&rb_link,&rb_parent);
> if (__vma && __vma->vm_start < vma->vm_end)
> return -ENOMEM;
> @@ -2677,3 +2677,10 @@ void __init mmap_init(void)
> ret = percpu_counter_init(&vm_committed_as, 0);
> VM_BUG_ON(ret);
> }
> +static int __init allksm(char *s)
> +{
> + randomize_va_space = 0;
> + vm_mergeable = VM_MERGEABLE;
> + return 1;
> +}
> +__setup("allksm", allksm);
> --
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--
Thanks,
//richard
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