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);
exactly for the case where it might have to split the pud entry.
Zapping the whole thing it does do without the assert.
I'm not going to swear the mmap_sem is absolutely required, since a
shared vma should be stable due to the i_mmap_lock, but splitting the
hugepage really is a fairly big deal.
It can't happen if you zap the *whole* mapping, but it can happen if
you have a start/end range. Like you do.
Also, in general it's probably not a great idea to look at
zap_page_range() (and copy_page_range()) for ideas.
They are kind of special, since they tend to be used for fundamental
whole-address-space operations (ie fork/exit) and so as a result they
get to do special things that a normal page walker generally shouldn't
do.
It's why they've never gotten translated to use the generic walker code.
Linus
