Re: [RFC v7 PATCH 4/4] mm: unmap special vmas with regular do_munmap()

[Yang Shi]

On 8/10/18 3:46 AM, Vlastimil Babka wrote:
> On 08/10/2018 01:36 AM, Yang Shi wrote:
> > Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or
> > have uprobes set, need get done with write mmap_sem held since
> > they may update vm_flags.
> >
> > So, it might be not safe enough to deal with these kind of special
> > mappings with read mmap_sem. Deal with such mappings with regular
> > do_munmap() call.
> >
> > Michal suggested to make this as a separate patch for safer and more
> > bisectable sake.
> >
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> >   mm/mmap.c | 24 ++++++++++++++++++++++++
> >   1 file changed, 24 insertions(+)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 2234d5a..06cb83c 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2766,6 +2766,16 @@ static inline void munlock_vmas(struct vm_area_struct *vma,
> >   	}
> >   }
> >   
> > +static inline bool can_zap_with_rlock(struct vm_area_struct *vma)
> > +{
> > +	if ((vma->vm_file &&
> > +	     vma_has_uprobes(vma, vma->vm_start, vma->vm_end)) |
> vma_has_uprobes() seems to be rather expensive check with e.g.
> unconditional spinlock. uprobe_munmap() seems to have some precondition
> cheaper checks for e.g. cases when there's no uprobes in the system
> (should be common?).

I think they are common, i.e. checking vm prot since uprobes are 
typically installed for VM_EXEC vmas. We could use those checks to save 
some cycles.

> BTW, uprobe_munmap() touches mm->flags, not vma->flags, so it should be
> evaluated more carefully for being called under mmap sem for reading, as
> having vmas already detached is no guarantee.

We might just leave uprobe vmas to use regular do_munmap? I'm supposed 
they should be not very common. And, uprobes just can be installed for 
VM_EXEC vma, although there may be large text segments, typically 
VM_EXEC vmas are unmapped when process exits, so the latency might be fine.

> > +	     (vma->vm_flags | (VM_HUGETLB | VM_PFNMAP)))
> 			    ^ I think replace '|' with '&' here?

Yes, thanks for catching this.

> > +		return false;
> > +
> > +	return true;
> > +}
> > +
> >   /*
> >    * Zap pages with read mmap_sem held
> >    *
> > @@ -2808,6 +2818,17 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
> >   			goto out;
> >   	}
> >   
> > +	/*
> > +	 * Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or
> > +	 * have uprobes set, need get done with write mmap_sem held since
> > +	 * they may update vm_flags. Deal with such mappings with regular
> > +	 * do_munmap() call.
> > +	 */
> > +	for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) {
> > +		if (!can_zap_with_rlock(vma))
> > +			goto regular_path;
> > +	}
> > +
> >   	/* Handle mlocked vmas */
> >   	if (mm->locked_vm) {
> >   		vma = start_vma;
> > @@ -2828,6 +2849,9 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start,
> >   
> >   	return 0;
> >   
> > +regular_path:
> I think it's missing a down_write_* here.

No, the jump is called before downgrade_write.

Thanks,
Yang

> > +	ret = do_munmap(mm, start, len, uf);
> > +
> >   out:
> >   	up_write(&mm->mmap_sem);
> >   	return ret;