Re: [PATCH] proc: pagemap: Hold mmap_sem during page walk

From: KAMEZAWA Hiroyuki
Date: Thu Apr 01 2010 - 01:59:09 EST


On Thu, 1 Apr 2010 14:54:43 +0900 (JST)
KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> > > So Matt, please actually address the _bug_ I pointed out rather than talk
> > > about other things. And yes, getting rid of the vma accesses sounds like
> > > it would fix it best. If that means that it doesn't work for hugepages, so
> > > be it.
> >
> > That'd actually take us back to where it was when it hit mainline, which
> > would make a lot of people unhappy. I wouldn't be one of them as there
> > thankfully aren't any huge pages in my world. But I'm convinced
> > put_user() must go. In which case, get_user_pages() stays, and I've got
> > to switch things to direct physical page access into that array.
>
> no. direct physical page access for /proc is completely wrong idea, i think.
> please imazine the caller process is multi threaded and it use fork case,
>
> example scenario)
> 1. the parent process has thread-A and thread-B.
> 2. thread-A call read_pagemap
> 3. read_pagemap grab the page-C
> 3. at the same time, thread-B call fork(), now page-C pointed from two process.
> 4. thread-B touch page-C, cow occur, then the parent process has cowed page (page-C')
> and the child process has original page-C.
> 5. thread-A write page-C by physical page access, then the child page is
> modified, instead parent one.
>
> I just recommend simply do double buffering.
>
Like this ?
CC'ed Horiguchi, he touched hugepage part of this code recently.

==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>

In initial design, walk_page_range() was designed just for walking page table and
it didn't require mmap_sem. Now, find_vma() etc.. are used in walk_page_range()
and we need mmap_sem around it.

This patch adds mmap_sem around walk_page_range().

Because /proc/<pid>/pagemap's callback routine use put_user(), we have to get
rid of it to do sane fix.

Reported-by: San Mehat <san@xxxxxxxxxx>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
---
fs/proc/task_mmu.c | 89 ++++++++++++++++++++++-------------------------------
1 file changed, 38 insertions(+), 51 deletions(-)

Index: linux-2.6.34-rc3/fs/proc/task_mmu.c
===================================================================
--- linux-2.6.34-rc3.orig/fs/proc/task_mmu.c
+++ linux-2.6.34-rc3/fs/proc/task_mmu.c
@@ -406,8 +406,11 @@ static int show_smap(struct seq_file *m,

memset(&mss, 0, sizeof mss);
mss.vma = vma;
- if (vma->vm_mm && !is_vm_hugetlb_page(vma))
+ if (vma->vm_mm && !is_vm_hugetlb_page(vma)) {
+ down_read(&vma->vm_mm->mmap_sem);
walk_page_range(vma->vm_start, vma->vm_end, &smaps_walk);
+ up_read(&vma->vm_mm->mmap_sem);
+ }

show_map_vma(m, vma);

@@ -552,7 +555,8 @@ const struct file_operations proc_clear_
};

struct pagemapread {
- u64 __user *out, *end;
+ int pos, len;
+ u64 *buffer;
};

#define PM_ENTRY_BYTES sizeof(u64)
@@ -575,10 +579,8 @@ struct pagemapread {
static int add_to_pagemap(unsigned long addr, u64 pfn,
struct pagemapread *pm)
{
- if (put_user(pfn, pm->out))
- return -EFAULT;
- pm->out++;
- if (pm->out >= pm->end)
+ pm->buffer[pm->pos++] = pfn;
+ if (pm->pos >= pm->len)
return PM_END_OF_BUFFER;
return 0;
}
@@ -720,21 +722,20 @@ static int pagemap_hugetlb_range(pte_t *
* determine which areas of memory are actually mapped and llseek to
* skip over unmapped regions.
*/
+#define PAGEMAP_WALK_SIZE (PMD_SIZE)
static ssize_t pagemap_read(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
struct task_struct *task = get_proc_task(file->f_path.dentry->d_inode);
- struct page **pages, *page;
- unsigned long uaddr, uend;
struct mm_struct *mm;
struct pagemapread pm;
- int pagecount;
int ret = -ESRCH;
struct mm_walk pagemap_walk = {};
unsigned long src;
unsigned long svpfn;
unsigned long start_vaddr;
unsigned long end_vaddr;
+ int copied = 0;

if (!task)
goto out;
@@ -757,34 +758,10 @@ static ssize_t pagemap_read(struct file
if (!mm)
goto out_task;

-
- uaddr = (unsigned long)buf & PAGE_MASK;
- uend = (unsigned long)(buf + count);
- pagecount = (PAGE_ALIGN(uend) - uaddr) / PAGE_SIZE;
- ret = 0;
- if (pagecount == 0)
+ pm.len = PM_ENTRY_BYTES * (PAGEMAP_WALK_SIZE >> PAGE_SHIFT);
+ pm.buffer = kmalloc(pm.len, GFP_KERNEL);
+ if (!pm.buffer)
goto out_mm;
- pages = kcalloc(pagecount, sizeof(struct page *), GFP_KERNEL);
- ret = -ENOMEM;
- if (!pages)
- goto out_mm;
-
- down_read(&current->mm->mmap_sem);
- ret = get_user_pages(current, current->mm, uaddr, pagecount,
- 1, 0, pages, NULL);
- up_read(&current->mm->mmap_sem);
-
- if (ret < 0)
- goto out_free;
-
- if (ret != pagecount) {
- pagecount = ret;
- ret = -EFAULT;
- goto out_pages;
- }
-
- pm.out = (u64 __user *)buf;
- pm.end = (u64 __user *)(buf + count);

pagemap_walk.pmd_entry = pagemap_pte_range;
pagemap_walk.pte_hole = pagemap_pte_hole;
@@ -807,23 +784,33 @@ static ssize_t pagemap_read(struct file
* user buffer is tracked in "pm", and the walk
* will stop when we hit the end of the buffer.
*/
- ret = walk_page_range(start_vaddr, end_vaddr, &pagemap_walk);
- if (ret == PM_END_OF_BUFFER)
- ret = 0;
- /* don't need mmap_sem for these, but this looks cleaner */
- *ppos += (char __user *)pm.out - buf;
- if (!ret)
- ret = (char __user *)pm.out - buf;
-
-out_pages:
- for (; pagecount; pagecount--) {
- page = pages[pagecount-1];
- if (!PageReserved(page))
- SetPageDirty(page);
- page_cache_release(page);
+ while (count && (start_vaddr < end_vaddr)) {
+ int len;
+ unsigned long end;
+ pm.pos = 0;
+ start_vaddr += PAGEMAP_WALK_SIZE;
+ end = start_vaddr + PAGEMAP_WALK_SIZE;
+ down_read(&mm->mmap_sem);
+ ret = walk_page_range(start_vaddr, end, &pagemap_walk);
+ up_read(&mm->mmap_sem);
+
+ len = PM_ENTRY_BYTES * pm.pos;
+ if (len > count)
+ len = count;
+ if (copy_to_user((char __user *)buf, pm.buffer, len) < 0) {
+ ret = -EFAULT;
+ goto out_free;
+ }
+ copied += len;
+ buf += len;
+ count -= len;
}
+ *ppos += copied;
+ if (!ret || ret == PM_END_OF_BUFFER)
+ ret = copied;
+
out_free:
- kfree(pages);
+ kfree(pm.buffer);
out_mm:
mmput(mm);
out_task:

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