Re: Ack to merge through DRM? WAS Re: [PATCH v2 1/5] mm: Add write-protect and clean utilities for address space ranges

[Kirill A. Shutemov]

On Thu, Sep 26, 2019 at 03:20:42PM -0700, Linus Torvalds wrote:
> On Thu, Sep 26, 2019 at 1:55 PM Thomas Hellström (VMware)
> <thomas_os@xxxxxxxxxxxx> wrote:
> >
> > Well, we're working on supporting huge puds and pmds in the graphics
> > VMAs, although in the write-notify cases we're looking at here, we would
> > probably want to split them down to PTE level.
> 
> Well, that's what the existing walker code does if you don't have that
> "pud_entry()" callback.
> 
> That said, I assume you would *not* want to do that if the huge
> pud/pmd is already clean and read-only, but just continue.
> 
> So you may want to have a special pud_entry() that handles that case.
> Eventually. Maybe. Although honestly, if you're doing dirty tracking,
> I doubt it makes much sense to use largepages.
> 
> > Looking at zap_pud_range() which when called from unmap_mapping_pages()
> > uses identical locking (no mmap_sem), it seems we should be able to get
> > away with i_mmap_lock(), making sure the whole page table doesn't
> > disappear under us. So it's not clear to me why the mmap_sem is strictly
> > needed here. Better to sort those restrictions out now rather than when
> > huge entries start appearing.
> 
> zap_pud_range()actually does have that
> 
>                VM_BUG_ON_VMA(!rwsem_is_locked(&tlb->mm->mmap_sem), vma);

The VM_BUG is a blind copy from PMD layer and it's bogus. i_mmap_lock()
works fine for file mappings.

The PMD was intended for THP case at the time when there were only
anon-THP. The check was relaxed and later dropped for file-THP on PMD
level. It has to be dropped on PUD too. We don't have anon-THP on PUD
level at all, only DAX played with them.
 
-- 
 Kirill A. Shutemov