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SubjectRe: [PATCH] proc: pagemap: Hold mmap_sem during page walk
Date
> > I just recommend simply do double buffering.
> >
> Like this ?
> CC'ed Horiguchi, he touched hugepage part of this code recently.

I like this patch. but I have few comment.

>
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> 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@google.com>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
> 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);
> + }

m_start already take mmap_sem. then I don't think this part is necessary.


>
> 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);

In /proc interface, GFP_TEMPORARY is recommened rather than GFP_KERNEL.
it help to prevent unnecessary external fragmentation.


> + 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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