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

[Thomas HellstrÃm (VMware)]

Hi,

On 9/26/19 9:19 PM, Linus Torvalds wrote:
> On Thu, Sep 26, 2019 at 5:03 AM Thomas HellstrÃm (VMware)
> <thomas_os@xxxxxxxxxxxx> wrote:
> > I wonder if I can get an ack from an mm maintainer to merge this through
> > DRM along with the vmwgfx patches? Andrew? Matthew?
> It would have helped to actually point to the patch itself, instead of
> just quoting the commit message.
>
> Looks like this:
>
>       https://lore.kernel.org/lkml/20190926115548.44000-2-thomas_os@xxxxxxxxxxxx/
>
> but why is the code in question not just using the regular page
> walkers. The commit log shows no explanation of what's so special
> about this?
>
> Is the only reason the locking magic? Because if that's the reason,
> then afaik we already have a function for that: it's
> __walk_page_range().
>
> Yes, it's static right now, but that's imho not a reason to duplicate
> all the walking (badly).
>
> Is there some other magic reason that isn't documented?
>
>                Linus

There is a discussion around this subject in

https://lore.kernel.org/lkml/20190926115548.44000-1-thomas_os@xxxxxxxxxxxx/

The main point is that there is an assert in pud_trans_huge_lock() that 
the mmap_sem is held, and we don't have it. Presumably we should be able 
to get away with the i_mmap_lock, but in addition I would have had to 
include the walk_as_pte_range() as the walk::pmd_entry anyway, so the 
amount of duplicated page walk code isn't that big.

That said, if people are OK with me modifying the assert in 
pud_trans_huge_lock() and make __walk_page_range non-static, it should 
probably be possible to make it work, yes.

Thanks,
Thomas