Re: [PATCH 4/4] add ksm kernel shared memory driver.

[Izik Eidus]

Anthony Liguori wrote:
> Izik Eidus wrote:
> > Ksm is driver that allow merging identical pages between one or more
> > applications in way unvisible to the application that use it.
> > Pages that are merged are marked as readonly and are COWed when any
> > application try to change them.
> >
> > Ksm is used for cases where using fork() is not suitable,
> > one of this cases is where the pages of the application keep changing
> > dynamicly and the application cannot know in advance what pages are
> > going to be identical.
> >
> > Ksm works by walking over the memory pages of the applications it
> > scan in order to find identical pages.
> > It uses a two sorted data strctures called stable and unstable trees
> > to find in effective way the identical pages.
> >
> > When ksm finds two identical pages, it marks them as readonly and merges
> > them into single one page,
> > after the pages are marked as readonly and merged into one page, linux
> > will treat this pages as normal copy_on_write pages and will fork them
> > when write access will happen to them.
> >
> > Ksm scan just memory areas that were registred to be scanned by it.
> >
> > Ksm api:
> >
> > KSM_GET_API_VERSION:
> > Give the userspace the api version of the module.
> >
> > KSM_CREATE_SHARED_MEMORY_AREA:
> > Create shared memory reagion fd, that latter allow the user to register
> > the memory region to scan by using:
> > KSM_REGISTER_MEMORY_REGION and KSM_REMOVE_MEMORY_REGION
> >
> > KSM_START_STOP_KTHREAD:
> > Return information about the kernel thread, the inforamtion is returned
> > using the ksm_kthread_info structure:
> > ksm_kthread_info:
> > __u32 sleep:
> >         number of microsecoends to sleep between each iteration of
> > scanning.
> >
> > __u32 pages_to_scan:
> >         number of pages to scan for each iteration of scanning.
> >
> > __u32 max_pages_to_merge:
> >         maximum number of pages to merge in each iteration of scanning
> >         (so even if there are still more pages to scan, we stop this
> > iteration)
> >
> > __u32 flags:
> >        flags to control ksmd (right now just ksm_control_flags_run
> >                   available)
> >   
>
> Wouldn't this make more sense as a sysfs interface?

I belive using ioctl for registering memory of applications make it 
easier....
Ksm doesnt have any complicated API that would benefit from sysfs 
(beside adding more complexity)

> That is, the KSM_START_STOP_KTHREAD part, not necessarily the rest of 
> the API.

What you mean?
> Regards,
>
> Anthony Liguori

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