[patch 0/6] Compound Page Enhancements

[patch 0/6] Compound Page Enhancements

[clameter]

This patch enhances the handling of compound pages in the VM. It may also
be important also for the antifrag patches that need to manage a set of
higher order free pages and also for other uses of compound pages.

For now it simplifies accounting for SLUB pages but the groundwork here is
important for the large block size patches and for allowing to page migration
of larger pages. With this framework we may be able to get to a point where
compound pages keep their flags while they are free and Mel may avoid having
special functions for determining the page order of higher order freed pages.
If we can avoid the setup and teardown of higher order pages then allocation
and release of compound pages will be faster.

Looking at the handling of compound pages we see that the fact that a page
is part of a higher order page is not that interesting. The differentiation
is mainly for head pages and tail pages of higher order pages. Head pages
usually need special handling to accomodate the larger size. It is usually
an error if tail pages are encountered. Or else they need to be treated
like PAGE_SIZE pages. So a compound flag in the page flags is not what we
need. Instead we introduce a flag for the head page and another for the tail
page. The PageCompound test is preserved for backward compatibility and
will test if either PageTail or PageHead has been set.

After this patchset the uses of CompoundPage() will be reduced significantly
in the core VM. The I/O layer will still use CompoundPage() for direct I/O.
However, if we at some point convert direct I/O to also support compound
pages as a single unit then CompoundPage() there may become unecessary as
well as the leftover check in mm/swap.c. We may end up mostly with checks
for PageTail and PageHead.

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[patch 1/6] Compound page handling enhancements

[clameter]

[-- Attachment #1: compound_headtail --]
[-- Type: text/plain, Size: 6597 bytes --]

This patch enhances the handling of compound pages in the VM. It may also
be important also for the antifrag patches that need to manage a set of
higher order free pages and also for other uses of compound pages.

For now it simplifies accounting for SLUB pages but the groundwork here is
important for the large block size patches and for allowing to page migration
of larger pages. With this framework we may be able to get to a point where
compound pages keep their flags while they are free and Mel may avoid having
special functions for determining the page order of higher order freed pages.
If we can avoid the setup and teardown of higher order pages then allocation
and release of compound pages will be faster.

Looking at the handling of compound pages we see that the fact that a page
is part of a higher order page is not that interesting. The differentiation
is mainly for head pages and tail pages of higher order pages. Head pages
usually need special handling to accomodate the larger size. It is usually
an error if tail pages are encountered. Or else they need to be treated
like PAGE_SIZE pages. So a compound flag in the page flags is not what we
need. Instead we introduce a flag for the head page and another for the tail
page. The PageCompound test is preserved for backward compatibility and
will test if either PageTail or PageHead has been set.

After this patchset the uses of CompoundPage() will be reduced significantly
in the core VM. The I/O layer will still use CompoundPage() for direct I/O.
However, if we at some point convert direct I/O to also support compound
pages as a single unit then CompoundPage() there may become unecessary as
well as the leftover check in mm/swap.c. We may end up mostly with checks
for PageTail and PageHead.

This patch:

Use two separate page flags for the head and tail of compound pages.
PageHead() and PageTail() become more efficient.

PageCompound then becomes a check for PageTail || PageHead. Over time
it is expected that PageCompound will mostly go away since the head page
processing will be different from tail page processing is most situations.

We can remove the compound page check from set_page_refcounted since
PG_reclaim is no longer overloaded.

Also the check in _free_one_page can only be for PageHead. We cannot
free a tail page.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/page-flags.h |   43 ++++++++++++-------------------------------
 mm/internal.h              |    2 +-
 mm/page_alloc.c            |    2 +-
 3 files changed, 14 insertions(+), 33 deletions(-)

Index: slub/include/linux/page-flags.h
===================================================================
--- slub.orig/include/linux/page-flags.h	2007-05-24 20:54:17.000000000 -0700
+++ slub/include/linux/page-flags.h	2007-05-24 21:12:21.000000000 -0700
@@ -83,7 +83,6 @@
 #define PG_private		11	/* If pagecache, has fs-private data */
 
 #define PG_writeback		12	/* Page is under writeback */
-#define PG_compound		13	/* Part of a compound page */
 #define PG_swapcache		14	/* Swap page: swp_entry_t in private */
 #define PG_mappedtodisk		15	/* Has blocks allocated on-disk */
 
@@ -92,6 +91,9 @@
 #define PG_lazyfree		18	/* MADV_FREE potential throwaway */
 #define PG_booked		19	/* Has blocks reserved on-disk */
 
+#define PG_head			21	/* Page is head of a compound page */
+#define PG_tail			22	/* Page is tail of a compound page */
+
 /* PG_readahead is only used for file reads; PG_reclaim is only for writes */
 #define PG_readahead		PG_reclaim /* Reminder to do async read-ahead */
 
@@ -227,37 +229,16 @@ static inline void SetPageUptodate(struc
 #define ClearPageLazyFree(page)	clear_bit(PG_lazyfree, &(page)->flags)
 #define __ClearPageLazyFree(page) __clear_bit(PG_lazyfree, &(page)->flags)
 
-#define PageCompound(page)	test_bit(PG_compound, &(page)->flags)
-#define __SetPageCompound(page)	__set_bit(PG_compound, &(page)->flags)
-#define __ClearPageCompound(page) __clear_bit(PG_compound, &(page)->flags)
-
-/*
- * PG_reclaim is used in combination with PG_compound to mark the
- * head and tail of a compound page
- *
- * PG_compound & PG_reclaim	=> Tail page
- * PG_compound & ~PG_reclaim	=> Head page
- */
-
-#define PG_head_tail_mask ((1L << PG_compound) | (1L << PG_reclaim))
-
-#define PageTail(page)	((page->flags & PG_head_tail_mask) \
-				== PG_head_tail_mask)
-
-static inline void __SetPageTail(struct page *page)
-{
-	page->flags |= PG_head_tail_mask;
-}
-
-static inline void __ClearPageTail(struct page *page)
-{
-	page->flags &= ~PG_head_tail_mask;
-}
+#define PageHead(page)		test_bit(PG_head, &(page)->flags)
+#define __SetPageHead(page)	__set_bit(PG_head, &(page)->flags)
+#define __ClearPageHead(page)	__clear_bit(PG_head, &(page)->flags)
+
+#define PageTail(page)		test_bit(PG_tail, &(page->flags))
+#define __SetPageTail(page)	__set_bit(PG_tail, &(page)->flags)
+#define __ClearPageTail(page)	__clear_bit(PG_tail, &(page)->flags)
 
-#define PageHead(page)	((page->flags & PG_head_tail_mask) \
-				== (1L << PG_compound))
-#define __SetPageHead(page)	__SetPageCompound(page)
-#define __ClearPageHead(page)	__ClearPageCompound(page)
+#define PageCompound(page)	((page)->flags & \
+				((1L << PG_head) | (1L << PG_tail)))
 
 #ifdef CONFIG_SWAP
 #define PageSwapCache(page)	test_bit(PG_swapcache, &(page)->flags)
Index: slub/mm/internal.h
===================================================================
--- slub.orig/mm/internal.h	2007-05-24 21:14:35.000000000 -0700
+++ slub/mm/internal.h	2007-05-24 21:16:14.000000000 -0700
@@ -24,7 +24,7 @@ static inline void set_page_count(struct
  */
 static inline void set_page_refcounted(struct page *page)
 {
-	VM_BUG_ON(PageCompound(page) && PageTail(page));
+	VM_BUG_ON(PageTail(page));
 	VM_BUG_ON(atomic_read(&page->_count));
 	set_page_count(page, 1);
 }
Index: slub/mm/page_alloc.c
===================================================================
--- slub.orig/mm/page_alloc.c	2007-05-24 21:32:12.000000000 -0700
+++ slub/mm/page_alloc.c	2007-05-24 21:32:17.000000000 -0700
@@ -449,7 +449,7 @@ static inline void __free_one_page(struc
 	int order_size = 1 << order;
 	int migratetype = get_pageblock_migratetype(page);
 
-	if (unlikely(PageCompound(page)))
+	if (unlikely(PageHead(page)))
 		destroy_compound_page(page, order);
 
 	page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1);

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[patch 2/6] compound pages: Add new support functions

[clameter]

[-- Attachment #1: compound_functions --]
[-- Type: text/plain, Size: 1344 bytes --]

compound_pages(page)	-> Determines base pages of a compound page

compound_shift(page)	-> Determine the page shift of a compound page

compound_size(page)	-> Determine the size of a compound page

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/mm.h |   15 +++++++++++++++
 1 file changed, 15 insertions(+)

Index: slub/include/linux/mm.h
===================================================================
--- slub.orig/include/linux/mm.h	2007-05-24 20:43:46.000000000 -0700
+++ slub/include/linux/mm.h	2007-05-24 20:50:51.000000000 -0700
@@ -366,6 +366,21 @@ static inline void set_compound_order(st
 	page[1].lru.prev = (void *)order;
 }
 
+static inline int compound_pages(struct page *page)
+{
+ 	return 1 << compound_order(page);
+}
+
+static inline int compound_shift(struct page *page)
+{
+ 	return PAGE_SHIFT + compound_order(page);
+}
+
+static inline int compound_size(struct page *page)
+{
+	return PAGE_SIZE << compound_order(page);
+}
+
 /*
  * Multiple processes may "see" the same page. E.g. for untouched
  * mappings of /dev/null, all processes see the same page full of

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[patch 3/6] compound pages: vmstat support

[clameter]

[-- Attachment #1: compound_vmstat --]
[-- Type: text/plain, Size: 2842 bytes --]

Add support for compound pages so that

inc_xxxx and dec_xxx

will increment the ZVCs by the number of pages of the compound page.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/vmstat.h |    5 ++---
 mm/vmstat.c            |   18 +++++++++++++-----
 2 files changed, 15 insertions(+), 8 deletions(-)

Index: slub/include/linux/vmstat.h
===================================================================
--- slub.orig/include/linux/vmstat.h	2007-05-24 20:37:44.000000000 -0700
+++ slub/include/linux/vmstat.h	2007-05-24 21:00:06.000000000 -0700
@@ -234,7 +234,7 @@ static inline void __inc_zone_state(stru
 static inline void __inc_zone_page_state(struct page *page,
 			enum zone_stat_item item)
 {
-	__inc_zone_state(page_zone(page), item);
+	__mod_zone_page_state(page_zone(page), item, compound_pages(page));
 }
 
 static inline void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
@@ -246,8 +246,7 @@ static inline void __dec_zone_state(stru
 static inline void __dec_zone_page_state(struct page *page,
 			enum zone_stat_item item)
 {
-	atomic_long_dec(&page_zone(page)->vm_stat[item]);
-	atomic_long_dec(&vm_stat[item]);
+	__mod_zone_page_state(page_zone(page), item, -compound_pages(page));
 }
 
 /*
Index: slub/mm/vmstat.c
===================================================================
--- slub.orig/mm/vmstat.c	2007-05-24 20:37:44.000000000 -0700
+++ slub/mm/vmstat.c	2007-05-24 21:00:06.000000000 -0700
@@ -224,7 +224,12 @@ void __inc_zone_state(struct zone *zone,
 
 void __inc_zone_page_state(struct page *page, enum zone_stat_item item)
 {
-	__inc_zone_state(page_zone(page), item);
+	struct zone *z = page_zone(page);
+
+	if (likely(!PageHead(page)))
+		__inc_zone_state(z, item);
+	else
+		__mod_zone_page_state(z, item, compound_pages(page));
 }
 EXPORT_SYMBOL(__inc_zone_page_state);
 
@@ -245,7 +250,12 @@ void __dec_zone_state(struct zone *zone,
 
 void __dec_zone_page_state(struct page *page, enum zone_stat_item item)
 {
-	__dec_zone_state(page_zone(page), item);
+	struct zone *z = page_zone(page);
+
+	if (likely(!PageHead(page)))
+		__dec_zone_state(z, item);
+	else
+		__mod_zone_page_state(z, item, -compound_pages(page));
 }
 EXPORT_SYMBOL(__dec_zone_page_state);
 
@@ -261,11 +271,9 @@ void inc_zone_state(struct zone *zone, e
 void inc_zone_page_state(struct page *page, enum zone_stat_item item)
 {
 	unsigned long flags;
-	struct zone *zone;
 
-	zone = page_zone(page);
 	local_irq_save(flags);
-	__inc_zone_state(zone, item);
+	__inc_zone_page_state(page, item);
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL(inc_zone_page_state);

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[patch 4/6] compound pages: Use new compound vmstat functions in SLUB

[clameter]

[-- Attachment #1: compound_vmstat_slub --]
[-- Type: text/plain, Size: 1765 bytes --]

Use the new dec/inc functions to simplify SLUB's accounting
of pages.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 mm/slub.c |   13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

Index: slub/mm/slub.c
===================================================================
--- slub.orig/mm/slub.c	2007-05-24 20:54:31.000000000 -0700
+++ slub/mm/slub.c	2007-05-24 21:03:45.000000000 -0700
@@ -965,7 +965,6 @@ static inline void kmem_cache_open_debug
 static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
 {
 	struct page * page;
-	int pages = 1 << s->order;
 
 	if (s->order)
 		flags |= __GFP_COMP;
@@ -984,10 +983,9 @@ static struct page *allocate_slab(struct
 	if (!page)
 		return NULL;
 
-	mod_zone_page_state(page_zone(page),
+	inc_zone_page_state(page,
 		(s->flags & SLAB_RECLAIM_ACCOUNT) ?
-		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
-		pages);
+		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE);
 
 	return page;
 }
@@ -1054,8 +1052,6 @@ out:
 
 static void __free_slab(struct kmem_cache *s, struct page *page)
 {
-	int pages = 1 << s->order;
-
 	if (unlikely(SlabDebug(page))) {
 		void *p;
 
@@ -1064,10 +1060,9 @@ static void __free_slab(struct kmem_cach
 			check_object(s, page, p, 0);
 	}
 
-	mod_zone_page_state(page_zone(page),
+	dec_zone_page_state(page,
 		(s->flags & SLAB_RECLAIM_ACCOUNT) ?
-		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE,
-		- pages);
+		NR_SLAB_RECLAIMABLE : NR_SLAB_UNRECLAIMABLE);
 
 	page->mapping = NULL;
 	__free_pages(page, s->order);

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[patch 5/6] compound pages: Allow use of get_page_unless_zero with compound pages

[clameter]

[-- Attachment #1: compound_get_one_unless --]
[-- Type: text/plain, Size: 1154 bytes --]

This will be needed by targeted slab reclaim in order to ensure that a
compound page allocated by SLUB will not go away under us.

It also may be needed if Mel starts to implement defragmentation. The
moving of compound pages may require the establishment of a reference
before the use of page migration functions.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 include/linux/mm.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: slub/include/linux/mm.h
===================================================================
--- slub.orig/include/linux/mm.h	2007-05-24 21:16:14.000000000 -0700
+++ slub/include/linux/mm.h	2007-05-24 21:16:45.000000000 -0700
@@ -293,7 +293,7 @@ static inline int put_page_testzero(stru
  */
 static inline int get_page_unless_zero(struct page *page)
 {
-	VM_BUG_ON(PageCompound(page));
+	VM_BUG_ON(PageTail(page));
 	return atomic_inc_not_zero(&page->_count);
 }
 

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[patch 6/6] compound pages: Allow freeing of compound pages via pagevec

[clameter]

[-- Attachment #1: compound_free_via_pagevec --]
[-- Type: text/plain, Size: 2638 bytes --]

Allow the freeing of compound pages via pagevec.

In release_pages() we currently special case for compound pages in order to
be sure to always decrement the page count of the head page and not the
tail page. However that redirection to the head page is only necessary for
tail pages. So use PageTail instead of PageCompound. No change therefore
for the handling of tail pages.

The head page of a compound pages now represents single page large page.
We do the usual processing including checking if its on the LRU
and removing it (not useful right now but later when compound pages are
on the LRU this will work). Then we add the compound page to the pagevec.
Only head pages will end up on the pagevec not tail pages.

In __pagevec_free() we then check if we are freeing a head page and if
so call the destructor for the compound page.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

---
 mm/page_alloc.c |   13 +++++++++++--
 mm/swap.c       |    8 +++++++-
 2 files changed, 18 insertions(+), 3 deletions(-)

Index: slub/mm/page_alloc.c
===================================================================
--- slub.orig/mm/page_alloc.c	2007-05-24 21:34:23.000000000 -0700
+++ slub/mm/page_alloc.c	2007-05-24 21:43:24.000000000 -0700
@@ -1760,8 +1760,17 @@ void __pagevec_free(struct pagevec *pvec
 {
 	int i = pagevec_count(pvec);
 
-	while (--i >= 0)
-		free_hot_cold_page(pvec->pages[i], pvec->cold);
+	while (--i >= 0) {
+		struct page *page = pvec->pages[i];
+
+		if (PageHead(page)) {
+			compound_page_dtor *dtor;
+
+			dtor = get_compound_page_dtor(page);
+			(*dtor)(page);
+		} else
+			free_hot_cold_page(page, pvec->cold);
+	}
 }
 
 fastcall void __free_pages(struct page *page, unsigned int order)
Index: slub/mm/swap.c
===================================================================
--- slub.orig/mm/swap.c	2007-05-24 21:34:23.000000000 -0700
+++ slub/mm/swap.c	2007-05-24 21:43:24.000000000 -0700
@@ -307,7 +307,13 @@ void release_pages(struct page **pages, 
 	for (i = 0; i < nr; i++) {
 		struct page *page = pages[i];
 
-		if (unlikely(PageCompound(page))) {
+		/*
+		 * If we have a tail page on the LRU then we need to
+		 * decrement the page count of the head page. There
+		 * is no further need to do anything since tail pages
+		 * cannot be on the LRU.
+		 */
+		if (unlikely(PageTail(page))) {
 			if (zone) {
 				spin_unlock_irq(&zone->lru_lock);
 				zone = NULL;

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Re: [patch 0/6] Compound Page Enhancements

[Andrew Morton]

On Thu, 24 May 2007 22:17:16 -0700 clameter@sgi.com wrote:

> This patch enhances the handling of compound pages in the VM. It may also
> be important also for the antifrag patches that need to manage a set of
> higher order free pages and also for other uses of compound pages.
> 
> For now it simplifies accounting for SLUB pages but the groundwork here is
> important for the large block size patches and for allowing to page migration
> of larger pages. With this framework we may be able to get to a point where
> compound pages keep their flags while they are free and Mel may avoid having
> special functions for determining the page order of higher order freed pages.
> If we can avoid the setup and teardown of higher order pages then allocation
> and release of compound pages will be faster.
> 
> Looking at the handling of compound pages we see that the fact that a page
> is part of a higher order page is not that interesting. The differentiation
> is mainly for head pages and tail pages of higher order pages. Head pages
> usually need special handling to accomodate the larger size. It is usually
> an error if tail pages are encountered. Or else they need to be treated
> like PAGE_SIZE pages. So a compound flag in the page flags is not what we
> need. Instead we introduce a flag for the head page and another for the tail
> page. The PageCompound test is preserved for backward compatibility and
> will test if either PageTail or PageHead has been set.
> 
> After this patchset the uses of CompoundPage() will be reduced significantly
> in the core VM. The I/O layer will still use CompoundPage() for direct I/O.
> However, if we at some point convert direct I/O to also support compound
> pages as a single unit then CompoundPage() there may become unecessary as
> well as the leftover check in mm/swap.c. We may end up mostly with checks
> for PageTail and PageHead.
> 

Well I've read that, and I've read the patches and I still don't see what
the point in all this is.

And looking back on it, I don't see the point in that PG_head_tail_mask
hack either.  We could have done

static inline int page_tail(struct page *page)
{
	return PageCompound(page) && (page->first_page != page);
}

Confused.  Don't know where this is all headed.

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Re: [patch 0/6] Compound Page Enhancements

[KAMEZAWA Hiroyuki]

On Thu, 24 May 2007 22:17:16 -0700
clameter@sgi.com wrote:

> This patch enhances the handling of compound pages in the VM. It may also
> be important also for the antifrag patches that need to manage a set of
> higher order free pages and also for other uses of compound pages.
> 
> For now it simplifies accounting for SLUB pages but the groundwork here is
> important for the large block size patches and for allowing to page migration
> of larger pages. With this framework we may be able to get to a point where
> compound pages keep their flags while they are free and Mel may avoid having
> special functions for determining the page order of higher order freed pages.
> If we can avoid the setup and teardown of higher order pages then allocation
> and release of compound pages will be faster.
> 

Keeping "free high order page" as "free compound page" in free_area[]-> and
avoid calling prep_compound_page() in page allocation ?

-Kame

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Re: [patch 0/6] Compound Page Enhancements

[Christoph Lameter]

On Fri, 25 May 2007, KAMEZAWA Hiroyuki wrote:

> Keeping "free high order page" as "free compound page" in free_area[]-> and
> avoid calling prep_compound_page() in page allocation ?

Right. That would be one benefit.

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Re: [patch 0/6] Compound Page Enhancements

[Christoph Lameter]

On Thu, 24 May 2007, Andrew Morton wrote:

> Well I've read that, and I've read the patches and I still don't see what
> the point in all this is.

A. vmstat can handle compound pages. No need to determine the size
   of a compound page to update VM statistics.

B. PageCompound is not useful in the long run. The processing of compound
   pages in the VM requires the knowledge if this is a head or tail page. That a 
   page is part of a compound page is not that useful knowlege.

C. Provide some more support for higher order page handling issues that
   Mel and I encounter.

D. Compound pages may be useful to handle higher order blocks on the 
   freelists if they can be handled efficiently. If we use the same 
   format for the freelist as for compound pages elsewhere then we will 
   have a common set of function to determine sizes etc etc.

E. Expands usefulness of get_page_unless_zero to compound pages. This
   is necessary to allow the moving of slabs and the moving of higher
   order pages for memory defragmentation.
 
F. Lays groundwork for large blocksize support.

> And looking back on it, I don't see the point in that PG_head_tail_mask
> hack either.  We could have done
> 
> static inline int page_tail(struct page *page)
> {
> 	return PageCompound(page) && (page->first_page != page);
> }
> 
> Confused.  Don't know where this is all headed.

To a better nicer looking VM.

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Re: [patch 0/6] Compound Page Enhancements

[Christoph Lameter]

On Thu, 24 May 2007, Andrew Morton wrote:

> And looking back on it, I don't see the point in that PG_head_tail_mask
> hack either.  We could have done
> 
> static inline int page_tail(struct page *page)
> {
> 	return PageCompound(page) && (page->first_page != page);
> }

But then PageHead(page) wont work anymore. pagehead->first_page is in use 
for some other purpose.

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Re: [patch 0/6] Compound Page Enhancements

[Andrew Morton]

On Fri, 25 May 2007 07:03:22 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:

> On Thu, 24 May 2007, Andrew Morton wrote:
> 
> > And looking back on it, I don't see the point in that PG_head_tail_mask
> > hack either.  We could have done
> > 
> > static inline int page_tail(struct page *page)
> > {
> > 	return PageCompound(page) && (page->first_page != page);
> > }
> 
> But then PageHead(page) wont work anymore. pagehead->first_page is in use 
> for some other purpose.

That's only because slub came along and screwed it all up.  The compound
page management used to be consistent, and simple.

Specifically: that lockless_freelist afterthought rendered us unable to fix
this mess.

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Re: [patch 0/6] Compound Page Enhancements

[Christoph Lameter]

On Fri, 25 May 2007, Andrew Morton wrote:

> > But then PageHead(page) wont work anymore. pagehead->first_page is in use 
> > for some other purpose.
> 
> That's only because slub came along and screwed it all up.  The compound
> page management used to be consistent, and simple.

Yeah I tried to keep it that way. Had to mess it up with the strange bit 
checks to get it in.

> Specifically: that lockless_freelist afterthought rendered us unable to fix
> this mess.

The main mess of page->private not being usable was fixed up. Now this can 
be even cleaner if we had PageTail and PageHead.

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