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

[Peter Xu <peterx@redhat.com>]

On Wed, May 19, 2021 at 08:49:01PM +1000, Alistair Popple wrote:
> 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.

Indeed, sorry for the false alarm!  Yes it would be great to mention that too.

-- 
Peter Xu