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SubjectRe: [PATCH][8/8] mm: lru interface change
Stone Wang wrote:
> Add Wired list to LRU.
> Add PG_Wired bit to page.
>

I may have missed something very trivial, but... why are they on a
list at all if they don't get scanned?


> diff -urN linux-2.6.15.orig/mm/swap.c linux-2.6.15/mm/swap.c
> --- linux-2.6.15.orig/mm/swap.c 2006-01-02 22:21:10.000000000 -0500
> +++ linux-2.6.15/mm/swap.c 2006-03-07 11:45:37.000000000 -0500
> @@ -110,6 +110,44 @@
> spin_unlock_irq(&zone->lru_lock);
> }
>
> +/* Wire the page; if the page is in LRU,
> + * try move it to Wired list.
> + */
> +void fastcall wire_page(struct page *page)

Ahh, here's the elusive beast.

> +{
> + struct zone *zone = page_zone(page);
> +
> + spin_lock_irq(&zone->lru_lock);

This is a fairly heavy lock for something which is going into page fault
fastpaths and such. You might be better off making wired_count atomic and
not using a lock in the common case if possible (even if it is only for
mlocked pages, I'm sure someone will complain).

> + page->wired_count ++;

Oh dear, I missed this change you made to struct page, tucked away in 5/8.
This alone pretty much makes it a showstopper, I'm afraid. You'll have to
work out some other way to do it so as not to penalise 99.999% of machines
which don't need this.

(Oh, and making the field a short usually won't help either, because of
alignment constraints).

> + if(!PageWired(page)){
> + if(PageLRU(page)){
> + del_page_from_lru(zone, page);
> + add_page_to_wired_list(zone,page);
> + SetPageWired(page);
> + }
> + }
> + spin_unlock_irq(&zone->lru_lock);
> +}
> +
> +/* Unwire the page.
> + * If it isnt wired by any process, try move it to active list.
> + */
> +void fastcall unwire_page(struct page *page)
> +{
> + struct zone *zone = page_zone(page);
> +
> + spin_lock_irq(&zone->lru_lock);
> + page->wired_count --;
> + if(!page->wired_count){
> + if(PageLRU(page) && TestClearPageWired(page)){
> + del_page_from_wired_list(zone,page);
> + add_page_to_active_list(zone,page);
> + SetPageActive(page);
> + }
> + }
> + spin_unlock_irq(&zone->lru_lock);
> +}
> +
> /*
> * Mark a page as having seen activity.
> *
> @@ -119,11 +157,13 @@
> */
> void fastcall mark_page_accessed(struct page *page)
> {
> - if (!PageActive(page) && PageReferenced(page) && PageLRU(page)) {
> - activate_page(page);
> - ClearPageReferenced(page);
> - } else if (!PageReferenced(page)) {
> - SetPageReferenced(page);
> + if(!PageWired(page)) {
> + if (!PageActive(page) && PageReferenced(page) && PageLRU(page)) {
> + activate_page(page);
> + ClearPageReferenced(page);
> + } else if (!PageReferenced(page)) {
> + SetPageReferenced(page);
> + }
> }
> }
>

So, umm... what happens when the page gets wired before activate_page()?

> @@ -178,13 +218,15 @@
> struct zone *zone = page_zone(page);
>
> spin_lock_irqsave(&zone->lru_lock, flags);
> - if (TestClearPageLRU(page))
> - del_page_from_lru(zone, page);
> - if (page_count(page) != 0)
> - page = NULL;
> + if(!PageWired(page)) {
> + if (TestClearPageLRU(page))
> + del_page_from_lru(zone, page);
> + if (page_count(page) != 0)
> + page = NULL;
> + if (page)
> + free_hot_page(page);
> + }
> spin_unlock_irqrestore(&zone->lru_lock, flags);
> - if (page)
> - free_hot_page(page);
> }
>

Hmm... how do PageWired pages get freed, then?

> EXPORT_SYMBOL(__page_cache_release);
> @@ -214,7 +256,8 @@
>
> if (!put_page_testzero(page))
> continue;
> -
> + if(PageWired(page))
> + continue;
> pagezone = page_zone(page);
> if (pagezone != zone) {
> if (zone)

Ditto.

--
SUSE Labs, Novell Inc.


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