Re: [PATCH v8 5/8] mm: Device exclusive memory access

[Alistair Popple]

On Wednesday, 19 May 2021 7:16:38 AM AEST Peter Xu wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Wed, Apr 07, 2021 at 06:42:35PM +1000, Alistair Popple wrote:
> 
> [...]
> 
> > +static bool try_to_protect(struct page *page, struct mm_struct *mm,
> > +                        unsigned long address, void *arg)
> > +{
> > +     struct ttp_args ttp = {
> > +             .mm = mm,
> > +             .address = address,
> > +             .arg = arg,
> > +             .valid = false,
> > +     };
> > +     struct rmap_walk_control rwc = {
> > +             .rmap_one = try_to_protect_one,
> > +             .done = page_not_mapped,
> > +             .anon_lock = page_lock_anon_vma_read,
> > +             .arg = &ttp,
> > +     };
> > +
> > +     /*
> > +      * Restrict to anonymous pages for now to avoid potential writeback
> > +      * issues.
> > +      */
> > +     if (!PageAnon(page))
> > +             return false;
> > +
> > +     /*
> > +      * During exec, a temporary VMA is setup and later moved.
> > +      * The VMA is moved under the anon_vma lock but not the
> > +      * page tables leading to a race where migration cannot
> > +      * find the migration ptes. Rather than increasing the
> > +      * locking requirements of exec(), migration skips
> > +      * temporary VMAs until after exec() completes.
> > +      */
> > +     if (!PageKsm(page) && PageAnon(page))
> > +             rwc.invalid_vma = invalid_migration_vma;
> > +
> > +     rmap_walk(page, &rwc);
> > +
> > +     return ttp.valid && !page_mapcount(page);
> > +}
> 
> I raised a question in the other thread regarding fork():
> 
> https://lore.kernel.org/lkml/YKQjmtMo+YQGx%2FwZ@t490s/
> 
> While I suddenly noticed that we may have similar issues even if we fork()
> before creating the ptes.
> 
> In that case, we may see multiple read-only ptes pointing to the same page. 
> We will convert all of them into device exclusive read ptes in rmap_walk()
> above, however how do we guarantee after all COW done in the parent and all
> the childs processes, the device owned page will be returned to the parent?

I assume you are talking about a fork() followed by a call to 
make_device_exclusive()? I think this should be ok because 
make_device_exclusive() always calls GUP with FOLL_WRITE both to break COW and 
because a device performing atomic operations needs to write to the page. I 
suppose a comment here highlighting the need to break COW to avoid this 
scenario would be useful though.

> E.g., if parent accesses the page earlier than the children processes
> (actually, as long as not the last one), do_wp_page() will do COW for parent
> on this page because refcount(page)>1, then the page seems to get lost to a
> random child too..
>
> To resolve all these complexity, not sure whether try_to_protect() could
> enforce VM_DONTCOPY (needs madvise MADV_DONTFORK in the user app), meanwhile
> make sure mapcount(page)==1 before granting the page to the device, so that
> this will guarantee this mm owns this page forever, I think?  It'll
> simplify fork() too as a side effect, since VM_DONTCOPY vma go away when
> fork.
> 
> --
> Peter Xu