Re: [RFC] Trylock the mmap_lock in vmf_anon_prepare()

[Matthew Wilcox <willy@infradead.org>]

On Mon, Apr 15, 2024 at 09:14:10AM -0700, Suren Baghdasaryan wrote:
> >         if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > -               vma_end_read(vma);
> > -               return VM_FAULT_RETRY;
> > +               if (!mmap_read_trylock(vma->vm_mm)) {
> > +                       vma_end_read(vma);
> > +                       return VM_FAULT_RETRY;
> > +               }
> >         }
> >         if (__anon_vma_prepare(vma))
> >                 return VM_FAULT_OOM;
> 
> You should drop mmap_lock when returning VM_FAULT_OOM as well.
> 
> > +       if (vmf->flags & FAULT_FLAG_VMA_LOCK)
> > +               mmap_read_unlock(vma->vm_mm);
> >         return 0;
> >  }

Thanks.  Fixed and pushed to
git://git.infradead.org/users/willy/pagecache.git vma-lock

+++ b/mm/memory.c
@@ -3224,16 +3224,21 @@ static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf)
 vm_fault_t vmf_anon_prepare(struct vm_fault *vmf)
 {
        struct vm_area_struct *vma = vmf->vma;
+       vm_fault_t ret = 0;

        if (likely(vma->anon_vma))
                return 0;
        if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
-               vma_end_read(vma);
-               return VM_FAULT_RETRY;
+               if (!mmap_read_trylock(vma->vm_mm)) {
+                       vma_end_read(vma);
+                       return VM_FAULT_RETRY;
+               }
        }
        if (__anon_vma_prepare(vma))
-               return VM_FAULT_OOM;
-       return 0;
+               ret = VM_FAULT_OOM;
+       if (vmf->flags & FAULT_FLAG_VMA_LOCK)
+               mmap_read_unlock(vma->vm_mm);
+       return ret;
 }

 /*