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SubjectRe: KSM kernel interface
On Sat, 19 Feb 2011, supercilious.dude@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).

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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