Do not account shmem pages to the page cache

Hi,

The appended patch does it's own accounting of shmem pages and adjust the page cache size to take these into account. So now again you will see shmem pages as used in top/vmstat etc. This confused a lot of people.

There is a uncertainty in the calculations since the vm may drop pages behind shmem and the number of shmem pages is estimated too high. This especially happens on truncate because first the page cache is reduced and later the shmem readjusts it's count.

To prevent negative cache sizes the adjustment is only done if shmem_nrpages > page_cache_size.

The latter part of the patch (all the init.c files) also exports the shmem page number to the shared memory field in meminfo. This means a change in semantics of this field but apparently a lot of people interpret this field exactly this way and it was not used any more

The patches are on top of my encapsulation patch.

Greetings
Christoph

diff -uNr 2.4.4-mSsu/fs/proc/proc_misc.c 2.4.4-mSsua/fs/proc/proc_misc.c

fs/proc/proc_misc.c +14 -3

@@ -140,6 +140,17 @@
 {
 	struct sysinfo i;
 	int len;
+	unsigned int cached, shmem;
+
+	/*
+	 * There may be some inconsistency because shmem_nrpages
+	 * update is delayed to page_cache_size
+	 * We make sure the cached value does not get below zero 
+	 */
+	cached = atomic_read(&page_cache_size);
+	shmem  = atomic_read(&shmem_nrpages);
+	if (shmem < cached)
+		cached -= shmem;
 
 /*
  * display in kilobytes.

@@ -153,8 +164,8 @@
                 "Swap: %8lu %8lu %8lu\n",
                 B(i.totalram), B(i.totalram-i.freeram), B(i.freeram),
                 B(i.sharedram), B(i.bufferram),
-                B(atomic_read(&page_cache_size)), B(i.totalswap),
-                B(i.totalswap-i.freeswap), B(i.freeswap));
+		B(cached), B(i.totalswap),
+		B(i.totalswap-i.freeswap), B(i.freeswap));
         /*
          * Tagged format, for easy grepping and expansion.
          * The above will go away eventually, once the tools

@@ -180,7 +191,7 @@
                 K(i.freeram),
                 K(i.sharedram),
                 K(i.bufferram),
-                K(atomic_read(&page_cache_size)),
+		K(cached),
 		K(nr_active_pages),
 		K(nr_inactive_dirty_pages),
 		K(nr_inactive_clean_pages()),

diff -uNr 2.4.4-mSsu/include/linux/shmem_fs.h 2.4.4-mSsua/include/linux/shmem_fs.h

include/linux/shmem_fs.h +2 -0

@@ -17,6 +17,8 @@
 	unsigned long val;
 } swp_entry_t;
 
+extern atomic_t shmem_nrpages;
+
 struct shmem_inode_info {
 	spinlock_t		lock;
 	struct semaphore 	sem;

diff -uNr 2.4.4-mSsu/mm/mmap.c 2.4.4-mSsua/mm/mmap.c

mm/mmap.c +13 -2

@@ -55,13 +55,24 @@
 	 */
 
 	long free;
-	
+	unsigned long cached, shmem;
+
+	/*
+	 * There may be some inconsistency because shmem_nrpages
+	 * update is delayed to the page_cache_size
+	 * We make sure the cached value does not get below zero 
+	 */
+	cached = atomic_read(&page_cache_size);
+	shmem  = atomic_read(&shmem_nrpages);
+	if (cached > shmem)
+		cached -= shmem;
+
         /* Sometimes we want to use more memory than we have. */
 	if (sysctl_overcommit_memory)
 	    return 1;
 
 	free = atomic_read(&buffermem_pages);
-	free += atomic_read(&page_cache_size);
+	free += cached;
 	free += nr_free_pages();
 	free += nr_swap_pages;
 

diff -uNr 2.4.4-mSsu/mm/shmem.c 2.4.4-mSsua/mm/shmem.c

mm/shmem.c +15 -6

@@ -3,7 +3,8 @@
  *
  * Copyright (C) 2000 Linus Torvalds.
  *		 2000 Transmeta Corp.
- *		 2000 Christoph Rohland
+ *		 2000-2001 Christoph Rohland
+ *		 2000-2001 SAP AG
  * 
  * This file is released under the GPL.
  */

@@ -45,6 +46,7 @@
 
 LIST_HEAD (shmem_inodes);
 static spinlock_t shmem_ilock = SPIN_LOCK_UNLOCKED;
+atomic_t shmem_nrpages = ATOMIC_INIT(0);
 
 #define BLOCKS_PER_PAGE (PAGE_SIZE/512)
 

@@ -52,6 +54,7 @@
  * shmem_recalc_inode - recalculate the size of an inode
  *
  * @inode: inode to recalc
+ * @swap:  additional swap pages freed externally
  *
  * We have to calculate the free blocks since the mm can drop pages
  * behind our back

@@ -62,12 +65,14 @@
  *
  * So the mm freed 
  * inodes->i_blocks/BLOCKS_PER_PAGE - 
- *			(inode->i_mapping->nrpages + info->swapped)
+ * 			(inode->i_mapping->nrpages + info->swapped)
  *
  * It has to be called with the spinlock held.
+ *
+ * The swap parameter is a performance hack for truncate.
  */
 
-static void shmem_recalc_inode(struct inode * inode)
+static void shmem_recalc_inode(struct inode * inode, unsigned long swap)
 {
 	unsigned long freed;
 

@@ -79,6 +84,7 @@
 		spin_lock (&info->stat_lock);
 		info->free_blocks += freed;
 		spin_unlock (&info->stat_lock);
+		atomic_sub(freed-swap, &shmem_nrpages);
 	}
 }
 

@@ -195,7 +201,7 @@
 out:
 	info->max_index = index;
 	info->swapped -= freed;
-	shmem_recalc_inode(inode);
+	shmem_recalc_inode(inode, freed);
 	spin_unlock (&info->lock);
 	up(&info->sem);
 }

@@ -250,14 +256,15 @@
 	entry = shmem_swp_entry(info, page->index);
 	if (IS_ERR(entry))	/* this had been allocted on page allocation */
 		BUG();
-	shmem_recalc_inode(page->mapping->host);
+	shmem_recalc_inode(page->mapping->host, 0);
 	error = -EAGAIN;
 	if (entry->val)
 		BUG();
 
 	*entry = swap;
 	error = 0;
-	/* Remove the from the page cache */
+	/* Remove the page from the page cache */
+	atomic_dec(&shmem_nrpages);
 	lru_cache_del(page);
 	remove_inode_page(page);
 

@@ -376,6 +383,7 @@
 	}
 
 	/* We have the page */
+	atomic_inc(&shmem_nrpages);
 	SetPageUptodate(page);
 	if (info->locked)
 		page_cache_get(page);

@@ -1275,6 +1283,7 @@
 	return 0;
 found:
 	add_to_page_cache(page, info->inode->i_mapping, offset + idx);
+	atomic_inc(&shmem_nrpages);
 	set_page_dirty(page);
 	SetPageUptodate(page);
 	UnlockPage(page);

diff -uNr 2.4.4-mSsu/arch/alpha/mm/init.c c/arch/alpha/mm/init.c

arch/alpha/mm/init.c +1 -1

@@ -402,7 +402,7 @@
 si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/arm/mm/init.c c/arch/arm/mm/init.c

arch/arm/mm/init.c +1 -1

@@ -647,7 +647,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram  = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram   = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/cris/mm/init.c c/arch/cris/mm/init.c

arch/cris/mm/init.c +1 -1

@@ -503,7 +503,7 @@
 
 	i = max_mapnr;
 	val->totalram = 0;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	while (i-- > 0)  {

diff -uNr 2.4.4-mSsu/arch/i386/mm/init.c c/arch/i386/mm/init.c

arch/i386/mm/init.c +1 -1

@@ -570,7 +570,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = totalhigh_pages;

diff -uNr 2.4.4-mSsu/arch/ia64/mm/init.c c/arch/ia64/mm/init.c

arch/ia64/mm/init.c +1 -1

@@ -151,7 +151,7 @@
 si_meminfo (struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/m68k/mm/init.c c/arch/m68k/mm/init.c

arch/m68k/mm/init.c +1 -1

@@ -217,7 +217,7 @@
 
     i = max_mapnr;
     val->totalram = totalram_pages;
-    val->sharedram = 0;
+    val->sharedram = atomic_read(&shmem_nrpages);
     val->freeram = nr_free_pages();
     val->bufferram = atomic_read(&buffermem_pages);
     while (i-- > 0) {

diff -uNr 2.4.4-mSsu/arch/mips/mm/init.c c/arch/mips/mm/init.c

arch/mips/mm/init.c +1 -1

@@ -343,7 +343,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/mips64/mm/init.c c/arch/mips64/mm/init.c

arch/mips64/mm/init.c +1 -1

@@ -411,7 +411,7 @@
 si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/parisc/mm/init.c c/arch/parisc/mm/init.c

arch/parisc/mm/init.c +1 -1

@@ -458,7 +458,7 @@
 
 	i = max_mapnr;
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 #if 0

diff -uNr 2.4.4-mSsu/arch/ppc/mm/init.c c/arch/ppc/mm/init.c

arch/ppc/mm/init.c +1 -1

@@ -336,7 +336,7 @@
 
 	i = max_mapnr;
 	val->totalram = 0;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	while (i-- > 0)  {

diff -uNr 2.4.4-mSsu/arch/s390/mm/init.c c/arch/s390/mm/init.c

arch/s390/mm/init.c +1 -1

@@ -271,7 +271,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/s390x/mm/init.c c/arch/s390x/mm/init.c

arch/s390x/mm/init.c +1 -1

@@ -284,7 +284,7 @@
 void si_meminfo(struct sysinfo *val)
 {
         val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = 0;

diff -uNr 2.4.4-mSsu/arch/sh/mm/init.c c/arch/sh/mm/init.c

arch/sh/mm/init.c +1 -1

@@ -215,7 +215,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = totalhigh_pages;

diff -uNr 2.4.4-mSsu/arch/sparc/mm/init.c c/arch/sparc/mm/init.c

arch/sparc/mm/init.c +1 -1

@@ -534,7 +534,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram = totalram_pages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 	val->totalhigh = totalhigh_pages;

diff -uNr 2.4.4-mSsu/arch/sparc64/mm/init.c c/arch/sparc64/mm/init.c

arch/sparc64/mm/init.c +1 -2

@@ -1512,7 +1512,7 @@
 void si_meminfo(struct sysinfo *val)
 {
 	val->totalram = num_physpages;
-	val->sharedram = 0;
+	val->sharedram = atomic_read(&shmem_nrpages);
 	val->freeram = nr_free_pages();
 	val->bufferram = atomic_read(&buffermem_pages);
 
 
-

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Hi,

The appended patch does it's own accounting of shmem pages and adjust the page cache size to take these into account. So now again you will see shmem pages as used in top/vmstat etc. This confused a lot of people.

There is a uncertainty in the calculations since the vm may drop pages behind shmem and the number of shmem pages is estimated too high. This especially happens on truncate because first the page cache is reduced and later the shmem readjusts it's count.

To prevent negative cache sizes the adjustment is only done if shmem_nrpages > page_cache_size.

The latter part of the patch (all the init.c files) also exports the shmem page number to the shared memory field in meminfo. This means a change in semantics of this field but apparently a lot of people interpret this field exactly this way and it was not used any more

The patches are on top of my encapsulation patch.

Greetings
Christoph

I have a dual ppro 200MHZ W6LI motherboard. I put 2.4.4-ac5 on last night, and the machine hung at Freeing unused Kernel memory. I selectively backed off what I thought were relevant patches. I got to aic7xxx, and ac5 without it worked. I attached /proc/scsi/aic7xxx/0.