[PATCH] Fix for vma merging refcounting bug

[PATCH] Fix for vma merging refcounting bug

[Stephen C. Tweedie]

When a new vma can be merged simultaneously with its two immediate
neighbours in both directions, vma_merge() extends the predecessor vma
and deletes the successor.  However, if the vma maps a file, it fails to
fput() when doing the delete, leaving the file's refcount inconsistent.

# This is a BitKeeper generated patch for the following project:
# Project Name: Linux kernel tree
# This patch format is intended for GNU patch command version 2.5 or higher.
# This patch includes the following deltas:
#	           ChangeSet	1.1083  -> 1.1084 
#	           mm/mmap.c	1.79    -> 1.80   
#
# The following is the BitKeeper ChangeSet Log
# --------------------------------------------
# 03/05/09	sct@sisko.scot.redhat.com	1.1084
# Fix vma merging problem leading to file refcount getting out of sync.
# --------------------------------------------
#
diff -Nru a/mm/mmap.c b/mm/mmap.c
--- a/mm/mmap.c	Fri May  9 13:26:53 2003
+++ b/mm/mmap.c	Fri May  9 13:26:53 2003
@@ -471,6 +471,8 @@
 			spin_unlock(lock);
 			if (need_up)
 				up(&inode->i_mapping->i_shared_sem);
+			if (file)
+				fput(file);
 
 			mm->map_count--;
 			kmem_cache_free(vm_area_cachep, next);


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Re: [PATCH] Fix for vma merging refcounting bug

[Andrea Arcangeli]

On Fri, May 09, 2003 at 01:34:21PM +0100, Stephen C. Tweedie wrote:
> When a new vma can be merged simultaneously with its two immediate
> neighbours in both directions, vma_merge() extends the predecessor vma
> and deletes the successor.  However, if the vma maps a file, it fails to
> fput() when doing the delete, leaving the file's refcount inconsistent.
> 
> # This is a BitKeeper generated patch for the following project:
> # Project Name: Linux kernel tree
> # This patch format is intended for GNU patch command version 2.5 or higher.
> # This patch includes the following deltas:
> #	           ChangeSet	1.1083  -> 1.1084 
> #	           mm/mmap.c	1.79    -> 1.80   
> #
> # The following is the BitKeeper ChangeSet Log
> # --------------------------------------------
> # 03/05/09	sct@sisko.scot.redhat.com	1.1084
> # Fix vma merging problem leading to file refcount getting out of sync.
> # --------------------------------------------
> #
> diff -Nru a/mm/mmap.c b/mm/mmap.c
> --- a/mm/mmap.c	Fri May  9 13:26:53 2003
> +++ b/mm/mmap.c	Fri May  9 13:26:53 2003
> @@ -471,6 +471,8 @@
>  			spin_unlock(lock);
>  			if (need_up)
>  				up(&inode->i_mapping->i_shared_sem);
> +			if (file)
> +				fput(file);
>  
>  			mm->map_count--;
>  			kmem_cache_free(vm_area_cachep, next);
> 

great catch! nobody could notice it in practice but it's definitely
needed, especially after long uptimes it could be noticeable ;), thanks!

I'm attaching for review what I'm applying to my -aa tree, to fix the
above and the other issue with the non-ram vma merging fixed in 2.5.

Please review, thanks again!

--- CGL/include/linux/mm.h.~1~	2003-05-07 23:39:00.000000000 +0200
+++ CGL/include/linux/mm.h	2003-05-10 18:25:04.000000000 +0200
@@ -587,11 +587,15 @@ static inline void __vma_unlink(struct m
 		mm->mmap_cache = prev;
 }
 
+#define VM_SPECIAL (VM_IO | VM_DONTCOPY | VM_DONTEXPAND | VM_RESERVED)
+
 #define can_vma_merge(vma, vm_flags) __can_vma_merge(vma, vm_flags, NULL, 0, 0)
 static inline int __can_vma_merge(struct vm_area_struct * vma, unsigned long vm_flags,
 				  struct file * file, unsigned long vm_pgoff, unsigned long offset)
 {
-	if (vma->vm_file == file && vma->vm_flags == vm_flags) {
+	if (vma->vm_file == file && vma->vm_flags == vm_flags &&
+	    likely((!vma->vm_ops || !vma->vm_ops->close) && !vma->vm_private_data &&
+		   !(vm_flags & VM_SPECIAL))) {
 		if (file) {
 			if (vma->vm_pgoff == vm_pgoff + offset) {
 				if ((long) offset > 0 && vm_pgoff + offset < vm_pgoff)
--- CGL/mm/mmap.c.~1~	2003-05-07 23:39:42.000000000 +0200
+++ CGL/mm/mmap.c	2003-05-10 18:25:23.000000000 +0200
@@ -377,6 +377,8 @@ static int vma_merge(struct mm_struct * 
 			spin_unlock(lock);
 			if (need_unlock)
 				unlock_vma_mappings(next);
+			if (file)
+				fput(file);
 
 			mm->map_count--;
 			kmem_cache_free(vm_area_cachep, next);

Andrea
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Re: [PATCH] Fix for vma merging refcounting bug

[Stephen C. Tweedie]

[-- Attachment #1: Type: text/plain, Size: 1001 bytes --]

Hi,

On Sat, 2003-05-10 at 17:33, Andrea Arcangeli wrote:
> On Fri, May 09, 2003 at 01:34:21PM +0100, Stephen C. Tweedie wrote:
> > When a new vma can be merged simultaneously with its two immediate
> > neighbours in both directions, vma_merge() extends the predecessor vma
> > and deletes the successor.  However, if the vma maps a file, it fails to
> > fput() when doing the delete, leaving the file's refcount inconsistent.

> great catch! nobody could notice it in practice

Yep --- I only noticed it because I was running a quick-and-dirty vma
merging test and wanted to test on a shmfs file, and noticed that the
temporary shmfs filesystem became unmountable afterwards.  Test
attached, in case anybody is interested (it's the third test, mapping a
file page by page in two interleaved passes, which triggers this case.)

> I'm attaching for review what I'm applying to my -aa tree, to fix the
> above and the other issue with the non-ram vma merging fixed in 2.5.

Looks OK.

Cheers,
 Stephen


[-- Attachment #2: vma-merge.c --]
[-- Type: text/x-c, Size: 2544 bytes --]

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>

#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/ipc.h>
#include <sys/shm.h>

static char *testfile = "/tmp/vma-test.dat";

#define TEST_PAGES 1024
int pagesize;
int filesize;

char *map_addr;

void DIE(char *) __attribute__ ((noreturn));

void DIE(char *why)
{
	perror(why);
	exit(1);
}

#define plural(n) (((n) == 1) ? "" : "s")
	
void test_maps(char *which)
{
	int fd;
	int rc;
	int count = 0;
	char buffer[256];
	char filename[128];
	FILE *mapfile;

	fd = open("/proc/self/maps", O_RDONLY);
	if (fd < 0)
		DIE("open(/proc/self/maps");


	mapfile = fopen("/proc/self/maps", "r");

	while (1) {
		if (!fgets(buffer, 256, mapfile))
			break;
		
		rc = sscanf(buffer, 
			    "%*x-%*x %*4s %*x %*5s %*d %127s\n", 
			    filename);
		if (!rc)
			continue;
		if (!strcmp(testfile, filename))
			count++;
	}
	
	printf("Testing %s: found %d map%s\n", which, count, plural(count));
}

#define clear_maps() \
	err = munmap(map_addr, filesize); 	\
	if (err)			\
		DIE("munmap");		\

static void map_page(int fd, int i)
{	
	char *ptr;

	ptr = mmap(map_addr + i * pagesize,
		   pagesize,
		   PROT_READ,
		   MAP_SHARED | MAP_FIXED,
		   fd,
		   i * pagesize);
	if (ptr == MAP_FAILED)
		DIE("mmap");
	if (ptr != map_addr + i * pagesize) {
		fprintf(stderr, "mmap returned unexpected address\n");
		exit(1);
	}
}

int main(int argc, char *argv[])
{
	int fd;
	int err;
	int i;

	if (argc > 1)
		testfile = argv[1];
	
	pagesize = getpagesize();
	filesize = TEST_PAGES * pagesize;

	fd = open(testfile, O_CREAT|O_TRUNC|O_RDWR, 0666);
	if (fd < 0)
		DIE("open");

	err = ftruncate(fd, filesize);
	if (err)
		DIE("ftuncate");

	/* Find a suitable mmap address for the entire file */
	map_addr = mmap(0, filesize, PROT_READ, MAP_SHARED, fd, 0);
	if (map_addr == MAP_FAILED)
		DIE("mmap");
	clear_maps();

	/* Now map it in piece by piece */
	for (i = 0; i < TEST_PAGES; i++)
		map_page(fd, i);
	test_maps("backwards merging");
	clear_maps();

	/* Next, map it in backwards */
	for (i = TEST_PAGES; i-- > 0; )
		map_page(fd, i);
	test_maps("forwards merging");
	clear_maps();

	/* Finally, map it in in two interleaved passes */
	for (i = 0; i < TEST_PAGES; i+=2)
		map_page(fd, i);
	for (i = 1; i < TEST_PAGES; i+=2)
		map_page(fd, i);
	test_maps("interleaved merging");

	close(fd);
	unlink(testfile);
	
	return 0;
}

Re: [PATCH] Fix for vma merging refcounting bug

[Andrea Arcangeli]

On Sun, May 11, 2003 at 09:04:06PM +0100, Stephen C. Tweedie wrote:
> Hi,
> 
> On Sat, 2003-05-10 at 17:33, Andrea Arcangeli wrote:
> > On Fri, May 09, 2003 at 01:34:21PM +0100, Stephen C. Tweedie wrote:
> > > When a new vma can be merged simultaneously with its two immediate
> > > neighbours in both directions, vma_merge() extends the predecessor vma
> > > and deletes the successor.  However, if the vma maps a file, it fails to
> > > fput() when doing the delete, leaving the file's refcount inconsistent.
> 
> > great catch! nobody could notice it in practice
> 
> Yep --- I only noticed it because I was running a quick-and-dirty vma
> merging test and wanted to test on a shmfs file, and noticed that the
> temporary shmfs filesystem became unmountable afterwards.  Test
> attached, in case anybody is interested (it's the third test, mapping a
> file page by page in two interleaved passes, which triggers this case.)
> 
> > I'm attaching for review what I'm applying to my -aa tree, to fix the
> > above and the other issue with the non-ram vma merging fixed in 2.5.
> 
> Looks OK.

actually I just noticed the fput is never been buggy in my tree:

	if (!file || !rb_parent || !vma_merge(mm, prev, rb_parent, addr, addr + len, vma->vm_flags, file, pgoff)) {
		vma_link(mm, vma, prev, rb_link, rb_parent);
		if (correct_wcount)
			atomic_inc(&file->f_dentry->d_inode->i_writecount);
	} else {
		if (file) {
			if (correct_wcount)
				atomic_inc(&file->f_dentry->d_inode->i_writecount);
			fput(file);
			^^^^^^^^^
		}
		kmem_cache_free(vm_area_cachep, vma);
	}

so this was a merging bug in 2.5

Andrea
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Re: [PATCH] Fix for vma merging refcounting bug

[Andrea Arcangeli]

On Wed, May 14, 2003 at 12:52:10AM +0200, Andrea Arcangeli wrote:
> On Sun, May 11, 2003 at 09:04:06PM +0100, Stephen C. Tweedie wrote:
> > Hi,
> > 
> > On Sat, 2003-05-10 at 17:33, Andrea Arcangeli wrote:
> > > On Fri, May 09, 2003 at 01:34:21PM +0100, Stephen C. Tweedie wrote:
> > > > When a new vma can be merged simultaneously with its two immediate
> > > > neighbours in both directions, vma_merge() extends the predecessor vma
> > > > and deletes the successor.  However, if the vma maps a file, it fails to
> > > > fput() when doing the delete, leaving the file's refcount inconsistent.
> > 
> > > great catch! nobody could notice it in practice
> > 
> > Yep --- I only noticed it because I was running a quick-and-dirty vma
> > merging test and wanted to test on a shmfs file, and noticed that the
> > temporary shmfs filesystem became unmountable afterwards.  Test
> > attached, in case anybody is interested (it's the third test, mapping a
> > file page by page in two interleaved passes, which triggers this case.)
> > 
> > > I'm attaching for review what I'm applying to my -aa tree, to fix the
> > > above and the other issue with the non-ram vma merging fixed in 2.5.
> > 
> > Looks OK.
> 
> actually I just noticed the fput is never been buggy in my tree:
> 
> 	if (!file || !rb_parent || !vma_merge(mm, prev, rb_parent, addr, addr + len, vma->vm_flags, file, pgoff)) {
> 		vma_link(mm, vma, prev, rb_link, rb_parent);
> 		if (correct_wcount)
> 			atomic_inc(&file->f_dentry->d_inode->i_writecount);
> 	} else {
> 		if (file) {
> 			if (correct_wcount)
> 				atomic_inc(&file->f_dentry->d_inode->i_writecount);
> 			fput(file);
> 			^^^^^^^^^
> 		}
> 		kmem_cache_free(vm_area_cachep, vma);
> 	}
> 
> so this was a merging bug in 2.5

Apologies, I was on the laptop reading that code wrong and I sent the
email too early, the patch I posted was correct for my tree too indeed.

Andrea
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