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

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Date:   Thu, 26 Sep 2019 15:20:42 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: Ack to merge through DRM? WAS Re: [PATCH v2 1/5] mm: Add
 write-protect and clean utilities for address space ranges

On Thu, Sep 26, 2019 at 1:55 PM Thomas Hellström (VMware)
<thomas_os@...pmail.org> 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