Messages in this thread | | | Date | Sat, 23 Aug 2014 19:15:57 -0400 | From | Peter Feiner <> | Subject | Re: [PATCH v2 1/3] mm: softdirty: enable write notifications on VMAs after VM_SOFTDIRTY cleared |
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On Sun, Aug 24, 2014 at 02:00:11AM +0300, Kirill A. Shutemov wrote: > On Sat, Aug 23, 2014 at 06:11:59PM -0400, Peter Feiner wrote: > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > > index dfc791c..f1a5382 100644 > > --- a/fs/proc/task_mmu.c > > +++ b/fs/proc/task_mmu.c > > @@ -851,8 +851,23 @@ static ssize_t clear_refs_write(struct file *file, const char __user *buf, > > if (type == CLEAR_REFS_MAPPED && !vma->vm_file) > > continue; > > if (type == CLEAR_REFS_SOFT_DIRTY) { > > - if (vma->vm_flags & VM_SOFTDIRTY) > > + if (vma->vm_flags & VM_SOFTDIRTY) { > > Why do we need the branch here. Does it save us anything? > Looks like we can update vm_flags and enable writenotify unconditionally. > Indentation level is high enough already.
You're right, we don't need the branch here. I'll change for v3.
> > vma->vm_flags &= ~VM_SOFTDIRTY; > > + /* > > + * We don't have a write lock on > > + * mm->mmap_sem, so we race with the > > + * fault handler reading vm_page_prot. > > + * Therefore writable PTEs (that won't > > + * have soft-dirty set) can be created > > + * for read faults. However, since the > > + * PTE lock is held while vm_page_prot > > + * is read and while we write protect > > + * PTEs during our walk, any writable > > + * PTEs that slipped through will be > > + * write protected. > > + */ > > Hm.. Isn't this yet another bug? > Updating vma->vm_flags without down_write(&mm->mmap_sem) looks troublesome > to me. Am I wrong?
As I said in the comment, it looks fishy but we're still fixing the bug. That is, no writable PTEs will sneak by that don't have soft-dirty set.
I was originally going to submit something that dropped the mmap_sem and re-took it in write mode before manipulating vm_page_prot. The control flow was slightly hairy, so I convinced myself that the race is benign :-)
If I'm right and the race is benign, it still might be worth having the more straightforward & obviously correct implementation since this isn't performance critical code.
> > +/* Enable write notifications without blowing away special flags. */ > > +static inline void vma_enable_writenotify(struct vm_area_struct *vma) > > +{ > > + vma->vm_page_prot = pgprot_modify(vma->vm_page_prot, > > + vm_get_page_prot(vma->vm_flags & > > + ~VM_SHARED)); > > I think this way is more readable: > > pgprot_t newprot; > newprot = vm_get_page_prot(vma->vm_flags & ~VM_SHARED); > vma->vm_page_prot = pgprot_modify(vma->vm_page_prot, newprot); >
Looks good. I'll update.
> > +} > > + > > +/* Disable write notifications without blowing away special flags. */ > > +static inline void vma_disable_writenotify(struct vm_area_struct *vma) > > +{ > > + vma->vm_page_prot = pgprot_modify(vma->vm_page_prot, > > + vm_get_page_prot(vma->vm_flags)); > > ditto.
I'll change this too.
Peter
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