Re: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb

[James Houghton]

On Thu, Jan 19, 2023 at 2:37 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Thu, Jan 05, 2023 at 10:18:33AM +0000, James Houghton wrote:
> > +     /*
> > +      * Grab the VMA lock and mapping sem for writing. This will prevent
> > +      * concurrent high-granularity page table walks, so that we can safely
> > +      * collapse and free page tables.
> > +      *
> > +      * This is the same locking that huge_pmd_unshare requires.
> > +      */
> > +     hugetlb_vma_lock_write(vma);
> > +     i_mmap_lock_write(vma->vm_file->f_mapping);
>
> One thing I just noticed - do we need the mmap write lock here?  I don't
> quickly see what stops another thread from having the mmap read and walking
> upon the pgtables being collapsed.

Maybe. Does huge_pmd_unshare() have the same problem?