Re: [RFC] memory hotremove patch take 2 [02/10] (make page unused)

On Thu, 10 May 2007 16:34:01 +0100 (IST)
Mel Gorman <mel@xxxxxxxxx> wrote:

+#ifdef CONFIG_PAGE_ISOLATION
+ /*
+ * For pages which are not used but not free.
+ * See include/linux/page_isolation.h
+ */
+ spinlock_t isolation_lock;
+ struct list_head isolation_list;
+#endif

Using MIGRATE_ISOLATING instead of this approach does mean that there will
be MAX_ORDER additional struct free_area added to the zone. That is more
lists than this approach.

Thank you!, its an interesting idea. I think it will make our code much
simpler. I'll look into.


I am somewhat suprised that CONFIG_PAGE_ISOLATION exists as a separate
option. If it was a compile-time option at all, I would expect it to
depend on memory hot-remove being selected.

I myself think CONFIG_PAGE_ISOLATION can be used by some code which need to
isolate some amount of contiguous pages. So config is divided for now.
Now, CONFIG_MEMORY_HOTREMOVE selects this.
CONFIG_PAGE_ISOLATION and CONFIG_MEMORY_HOTREMOVE will be merged later
if there are no one who use this except for hot-removal.



/*
* zone_start_pfn, spanned_pages and present_pages are all
* protected by span_seqlock. It is a seqlock because it has
Index: current_test/mm/page_alloc.c
===================================================================
--- current_test.orig/mm/page_alloc.c 2007-05-08 15:07:20.000000000 +0900
+++ current_test/mm/page_alloc.c 2007-05-08 15:08:34.000000000 +0900
@@ -41,6 +41,7 @@
#include <linux/pfn.h>
#include <linux/backing-dev.h>
#include <linux/fault-inject.h>
+#include <linux/page_isolation.h>

#include <asm/tlbflush.h>
#include <asm/div64.h>
@@ -448,6 +449,9 @@ static inline void __free_one_page(struc
if (unlikely(PageCompound(page)))
destroy_compound_page(page, order);

+ if (page_under_isolation(zone, page, order))
+ return;
+

Using MIGRATE_ISOLATING would avoid a potential list search here.

yes. thank you.

page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1);

VM_BUG_ON(page_idx & (order_size - 1));
@@ -3259,6 +3263,10 @@ static void __meminit free_area_init_cor
zone->nr_scan_inactive = 0;
zap_zone_vm_stats(zone);
atomic_set(&zone->reclaim_in_progress, 0);
+#ifdef CONFIG_PAGE_ISOLATION
+ spin_lock_init(&zone->isolation_lock);
+ INIT_LIST_HEAD(&zone->isolation_list);
i> +#endif
if (!size)
continue;

@@ -4214,3 +4222,182 @@ void set_pageblock_flags_group(struct pa
else
__clear_bit(bitidx + start_bitidx, bitmap);
}
+
+#ifdef CONFIG_PAGE_ISOLATION
+/*
+ * Page Isolation.
+ *
+ * If a page is removed from usual free_list and will never be used,
+ * It is linked to "struct isolation_info" and set Reserved, Private
+ * bit. page->mapping points to isolation_info in it.
+ * and page_count(page) is 0.
+ *
+ * This can be used for creating a chunk of contiguous *unused* memory.
+ *
+ * current user is Memory-Hot-Remove.
+ * maybe move to some other file is better.

page_isolation.c to match the header filename seems reasonable.
page_alloc.c has a lot of multi-function stuff like memory initialisation
in it.

Hmm.


+ */
+static void
+isolate_page_nolock(struct isolation_info *info, struct page *page, int order)
+{
+ int pagenum;
+ pagenum = 1 << order;
+ while (pagenum > 0) {
+ SetPageReserved(page);
+ SetPagePrivate(page);
+ page->private = (unsigned long)info;
+ list_add(&page->lru, &info->pages);
+ page++;
+ pagenum--;
+ }
+}

It's worth commenting somewhere that pages on the list in isolation_info
are always order-0.

okay.

+
+/*
+ * This function is called from page_under_isolation()
+ */
+
+int __page_under_isolation(struct zone *zone, struct page *page, int order)
+{
+ struct isolation_info *info;
+ unsigned long pfn = page_to_pfn(page);
+ unsigned long flags;
+ int found = 0;
+
+ spin_lock_irqsave(&zone->isolation_lock,flags);

An unwritten convention seems to be that __ versions of same-named
functions are the nolock version. i.e. I would expect
page_under_isolation() to acquire and release the spinlock and
__page_under_isolation() to do no additional locking.

Locking outside of here might make the flow a little clearer as well if
you had two returns and avoided the use of "found".

Maybe MOVABLE_ISOLATING will simplify these code.


+ list_for_each_entry(info, &zone->isolation_list, list) {
+ if (info->start_pfn <= pfn && pfn < info->end_pfn) {
+ found = 1;
+ break;
+ }
+ }
+ if (found) {
+ isolate_page_nolock(info, page, order);
+ }
+ spin_unlock_irqrestore(&zone->isolation_lock, flags);
+ return found;
+}
+
+/*
+ * start and end must be in the same zone.
+ *
+ */
+struct isolation_info *
+register_isolation(unsigned long start, unsigned long end)
+{
+ struct zone *zone;
+ struct isolation_info *info = NULL, *tmp;
+ unsigned long flags;
+ unsigned long last_pfn = end - 1;
+
+ if (!pfn_valid(start) || !pfn_valid(last_pfn) || (start >= end))
+ return ERR_PTR(-EINVAL);
+ /* check start and end is in the same zone */
+ zone = page_zone(pfn_to_page(start));
+
+ if (zone != page_zone(pfn_to_page(last_pfn)))
+ return ERR_PTR(-EINVAL);
+ /* target range has to match MAX_ORDER alignmet */
+ if ((start & (MAX_ORDER_NR_PAGES - 1)) ||
+ (end & (MAX_ORDER_NR_PAGES - 1)))
+ return ERR_PTR(-EINVAL);

Why does the range have to be MAX_ORDER alighned?

+ info = kmalloc(sizeof(*info), GFP_KERNEL);
+ if (!info)
+ return ERR_PTR(-ENOMEM);
+ spin_lock_irqsave(&zone->isolation_lock, flags);
+ /* we don't allow overlap among isolation areas */
+ if (!list_empty(&zone->isolation_list)) {
+ list_for_each_entry(tmp, &zone->isolation_list, list) {
+ if (start < tmp->end_pfn && end > tmp->start_pfn) {
+ goto out_free;
+ }
+ }
+ }

Why not merge requests for overlapping isolations?

This is related to memory-unplug interface. It doesn't allow overlaping.
So this is not expected to happen. just sanity check.
but this code will be removed by MIGRATE_ISOLATING.

Thank you for your good idea.

-Kame



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