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SubjectRe: [PATCH 2/2 V2] ksm: take dirty bit as reference to avoid volatile pages scanning
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(Sorry for repeated mail, I forgot to Cc the list..)

On Wednesday 22 June 2011 06:38:00 you wrote:
> * Nai Xia (nai.xia@gmail.com) wrote:
> > Introduced ksm_page_changed() to reference the dirty bit of a pte. We clear
> > the dirty bit for each pte scanned but don't flush the tlb. For a huge page,
> > if one of the subpage has changed, we try to skip the whole huge page
> > assuming(this is true by now) that ksmd linearly scans the address space.
>
> This doesn't build w/ kvm as a module.

I think it's because of the name-error of a related kvm patch, which I only sent
in a same email thread. http://marc.info/?l=linux-mm&m=130866318804277&w=2
The patch split is not clean...I'll redo it.

>
> > A NEW_FLAG is also introduced as a status of rmap_item to make ksmd scan
> > more aggressively for new VMAs - only skip the pages considered to be volatile
> > by the dirty bits. This can be enabled/disabled through KSM's sysfs interface.
>
> This seems like it should be separated out. And while it might be useful
> to enable/disable for testing, I don't think it's worth supporting for
> the long term. Would also be useful to see the value of this flag.

I think it maybe useful for uses who want to turn on/off this scan policy explicitly
according to their working sets?

>
> > @@ -454,7 +468,7 @@ static void remove_node_from_stable_tree(struct stable_node *stable_node)
> > else
> > ksm_pages_shared--;
> > put_anon_vma(rmap_item->anon_vma);
> > - rmap_item->address &= PAGE_MASK;
> > + rmap_item->address &= ~STABLE_FLAG;
> > cond_resched();
> > }
> >
> > @@ -542,7 +556,7 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
> > ksm_pages_shared--;
> >
> > put_anon_vma(rmap_item->anon_vma);
> > - rmap_item->address &= PAGE_MASK;
> > + rmap_item->address &= ~STABLE_FLAG;
> >
> > } else if (rmap_item->address & UNSTABLE_FLAG) {
> > unsigned char age;
> > @@ -554,12 +568,14 @@ static void remove_rmap_item_from_tree(struct rmap_item *rmap_item)
> > * than left over from before.
> > */
> > age = (unsigned char)(ksm_scan.seqnr - rmap_item->address);
> > - BUG_ON(age > 1);
> > + BUG_ON (age > 1);
>
> No need to add space after BUG_ON() there
>
> > +
> > if (!age)
> > rb_erase(&rmap_item->node, &root_unstable_tree);
> >
> > ksm_pages_unshared--;
> > - rmap_item->address &= PAGE_MASK;
> > + rmap_item->address &= ~UNSTABLE_FLAG;
> > + rmap_item->address &= ~SEQNR_MASK;
>
> None of these changes are needed AFAICT. &= PAGE_MASK clears all
> relevant bits. How could it be in a tree, have NEW_FLAG set, and
> while removing from tree want to preserve NEW_FLAG?

You are right, it's meaningless to preserve NEW_FLAG after it goes
through the trees. I'll revert the lines.

Thanks!

Nai

>
> thanks,
> -chris
>


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