linux-kernel - Re: KSM kernel interface

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Message-ID: <AANLkTimer0oq8hfb2wQs5PxvTVZcnqXLo9N-PZ9wtGH2@mail.gmail.com>
Date:	Sat, 19 Feb 2011 20:22:21 +0100
From:	richard -rw- weinberger <richard.weinberger@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	supercilious.dude@...il.com, linux-kernel@...r.kernel.org,
	lxc-devel@...ts.sourceforge.net
Subject: Re: KSM kernel interface

On Sat, Feb 19, 2011 at 7:54 PM, Hugh Dickins <hughd@...gle.com> wrote:
> On Sat, 19 Feb 2011, supercilious.dude@...il.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(&current->mm->mmap_sem).
> - */
> -
> + * The caller must hold down_write(&current->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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