Re: [PATCH v5 3.1.0-rc4-tip 4/26] uprobes: Define hooks for mmap/munmap.

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Peter Zijlstra <peterz@infradead.org> [2011-09-27 13:37:15]:

> On Mon, 2011-09-26 at 21:14 +0530, Srikar Dronamraju wrote:
> > 
> > > > +
> > > > +/*
> > > > + * Called from mmap_region.
> > > > + * called with mm->mmap_sem acquired.
> > > > + *
> > > > + * Return -ve no if we fail to insert probes and we cannot
> > > > + * bail-out.
> > > > + * Return 0 otherwise. i.e :
> > > > + * - successful insertion of probes
> > > > + * - (or) no possible probes to be inserted.
> > > > + * - (or) insertion of probes failed but we can bail-out.
> > > > + */
> > > > +int mmap_uprobe(struct vm_area_struct *vma)
> > > > +{
> > > > +   struct list_head tmp_list;
> > > > +   struct uprobe *uprobe, *u;
> > > > +   struct inode *inode;
> > > > +   int ret = 0;
> > > > +
> > > > +   if (!valid_vma(vma))
> > > > +           return ret;     /* Bail-out */
> > > > +
> > > > +   inode = igrab(vma->vm_file->f_mapping->host);
> > > > +   if (!inode)
> > > > +           return ret;
> > > > +
> > > > +   INIT_LIST_HEAD(&tmp_list);
> > > > +   mutex_lock(&uprobes_mmap_mutex);
> > > > +   build_probe_list(inode, &tmp_list);
> > > > +   list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
> > > > +           loff_t vaddr;
> > > > +
> > > > +           list_del(&uprobe->pending_list);
> > > > +           if (!ret && uprobe->consumers) {
> > > > +                   vaddr = vma->vm_start + uprobe->offset;
> > > > +                   vaddr -= vma->vm_pgoff << PAGE_SHIFT;
> > > > +                   if (vaddr < vma->vm_start || vaddr >= vma->vm_end)
> > > > +                           continue;
> > > > +                   ret = install_breakpoint(vma->vm_mm, uprobe);
> > > > +
> > > > +                   if (ret && (ret == -ESRCH || ret == -EEXIST))
> > > > +                           ret = 0;
> > > > +           }
> > > > +           put_uprobe(uprobe);
> > > > +   }
> > > > +
> > > > +   mutex_unlock(&uprobes_mmap_mutex);
> > > > +   iput(inode);
> > > > +   return ret;
> > > > +}
> > > > +
> > > > +static void dec_mm_uprobes_count(struct vm_area_struct *vma,
> > > > +           struct inode *inode)
> > > > +{
> > > > +   struct uprobe *uprobe;
> > > > +   struct rb_node *n;
> > > > +   unsigned long flags;
> > > > +
> > > > +   n = uprobes_tree.rb_node;
> > > > +   spin_lock_irqsave(&uprobes_treelock, flags);
> > > > +   uprobe = __find_uprobe(inode, 0, &n);
> > > > +
> > > > +   /*
> > > > +    * If indeed there is a probe for the inode and with offset zero,
> > > > +    * then lets release its reference. (ref got thro __find_uprobe)
> > > > +    */
> > > > +   if (uprobe)
> > > > +           put_uprobe(uprobe);
> > > > +   for (; n; n = rb_next(n)) {
> > > > +           loff_t vaddr;
> > > > +
> > > > +           uprobe = rb_entry(n, struct uprobe, rb_node);
> > > > +           if (uprobe->inode != inode)
> > > > +                   break;
> > > > +           vaddr = vma->vm_start + uprobe->offset;
> > > > +           vaddr -= vma->vm_pgoff << PAGE_SHIFT;
> > > > +           if (vaddr < vma->vm_start || vaddr >= vma->vm_end)
> > > > +                   continue;
> > > > +           atomic_dec(&vma->vm_mm->mm_uprobes_count);
> > > > +   }
> > > > +   spin_unlock_irqrestore(&uprobes_treelock, flags);
> > > > +}
> > > > +
> > > > +/*
> > > > + * Called in context of a munmap of a vma.
> > > > + */
> > > > +void munmap_uprobe(struct vm_area_struct *vma)
> > > > +{
> > > > +   struct inode *inode;
> > > > +
> > > > +   if (!valid_vma(vma))
> > > > +           return;         /* Bail-out */
> > > > +
> > > > +   if (!atomic_read(&vma->vm_mm->mm_uprobes_count))
> > > > +           return;
> > > > +
> > > > +   inode = igrab(vma->vm_file->f_mapping->host);
> > > > +   if (!inode)
> > > > +           return;
> > > > +
> > > > +   dec_mm_uprobes_count(vma, inode);
> > > > +   iput(inode);
> > > > +   return;
> > > > +}
> > > 
> > > One has to wonder why mmap_uprobe() can be one function but
> > > munmap_uprobe() cannot.
> > > 
> > 
> > I didnt understand this comment, Can you please elaborate?
> > mmap_uprobe uses build_probe_list and munmap_uprobe uses
> > dec_mm_uprobes_count. 
> 
> Ah, I missed build_probe_list(), but I didn't see a reason for the
> existence of dec_mm_uprobe_count(), the name doesn't make sense and the
> content is 'small' enough to just put in munmap_uprobe.
> 
> To me it looks similar to the list iteration you have in mmap_uprobe(),
> you didn't split that out into another function either.

Hmm, For me whats done in dec_mm_uprobe_count() is similar to whats
done in build_probe_list(), just that build_probe_list does a atomic_inc
+ list_add while dec_mm_uprobe_count() does a atomic_dec.

When I kept dec_mm_uprobe_count() inside munmap_uprobe(), I found most
of the code nested too deep. Hence carved it out as a separate function.

I open to suggestions for dec_mm_uprobe_count()

-- 
Thanks and Regards
Srikar