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SubjectRe: [PATCH 1/7] ksm: add some comments
On 02/21/2013 04:19 PM, Hugh Dickins wrote:
> Added slightly more detail to the Documentation of merge_across_nodes,
> a few comments in areas indicated by review, and renamed get_ksm_page()'s
> argument from "locked" to "lock_it". No functional change.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
> Documentation/vm/ksm.txt | 16 ++++++++++++----
> mm/ksm.c | 18 ++++++++++++++----
> 2 files changed, 26 insertions(+), 8 deletions(-)
>
> --- mmotm.orig/Documentation/vm/ksm.txt 2013-02-20 22:28:09.456001057 -0800
> +++ mmotm/Documentation/vm/ksm.txt 2013-02-20 22:28:23.580001392 -0800
> @@ -60,10 +60,18 @@ sleep_millisecs - how many milliseconds
>
> merge_across_nodes - specifies if pages from different numa nodes can be merged.
> When set to 0, ksm merges only pages which physically
> - reside in the memory area of same NUMA node. It brings
> - lower latency to access to shared page. Value can be
> - changed only when there is no ksm shared pages in system.
> - Default: 1
> + reside in the memory area of same NUMA node. That brings
> + lower latency to access of shared pages. Systems with more
> + nodes, at significant NUMA distances, are likely to benefit
> + from the lower latency of setting 0. Smaller systems, which
> + need to minimize memory usage, are likely to benefit from
> + the greater sharing of setting 1 (default). You may wish to
> + compare how your system performs under each setting, before
> + deciding on which to use. merge_across_nodes setting can be
> + changed only when there are no ksm shared pages in system:
> + set run 2 to unmerge pages first, then to 1 after changing
> + merge_across_nodes, to remerge according to the new setting.

What's the root reason merge_across_nodes setting just can be changed
only when there are no ksm shared pages in system? Can they be unmerged
and merged again during ksmd scan?

> + Default: 1 (merging across nodes as in earlier releases)
>
> run - set 0 to stop ksmd from running but keep merged pages,
> set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
> --- mmotm.orig/mm/ksm.c 2013-02-20 22:28:09.456001057 -0800
> +++ mmotm/mm/ksm.c 2013-02-20 22:28:23.584001392 -0800
> @@ -87,6 +87,9 @@
> * take 10 attempts to find a page in the unstable tree, once it is found,
> * it is secured in the stable tree. (When we scan a new page, we first
> * compare it against the stable tree, and then against the unstable tree.)
> + *
> + * If the merge_across_nodes tunable is unset, then KSM maintains multiple
> + * stable trees and multiple unstable trees: one of each for each NUMA node.
> */
>
> /**
> @@ -524,7 +527,7 @@ static void remove_node_from_stable_tree
> * a page to put something that might look like our key in page->mapping.
> * is on its way to being freed; but it is an anomaly to bear in mind.
> */
> -static struct page *get_ksm_page(struct stable_node *stable_node, bool locked)
> +static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
> {
> struct page *page;
> void *expected_mapping;
> @@ -573,7 +576,7 @@ again:
> goto stale;
> }
>
> - if (locked) {
> + if (lock_it) {
> lock_page(page);
> if (ACCESS_ONCE(page->mapping) != expected_mapping) {
> unlock_page(page);
> @@ -703,10 +706,17 @@ static int remove_stable_node(struct sta
> return 0;
> }
>
> - if (WARN_ON_ONCE(page_mapped(page)))
> + if (WARN_ON_ONCE(page_mapped(page))) {
> + /*
> + * This should not happen: but if it does, just refuse to let
> + * merge_across_nodes be switched - there is no need to panic.
> + */
> err = -EBUSY;
> - else {
> + } else {
> /*
> + * The stable node did not yet appear stale to get_ksm_page(),
> + * since that allows for an unmapped ksm page to be recognized
> + * right up until it is freed; but the node is safe to remove.
> * This page might be in a pagevec waiting to be freed,
> * or it might be PageSwapCache (perhaps under writeback),
> * or it might have been removed from swapcache a moment ago.
>
> --
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