Re: [PATCH] dma-resv: lockdep-prime address_space->i_mmap_rwsem for dma-resv

Daniel Vetter
2020-07-30T13:17:32Z

On Thu, Jul 30, 2020 at 2:17 PM Thomas Hellström (Intel)
<thomas_os@shipmail.org> wrote:
>
>
> On 7/28/20 3:58 PM, Daniel Vetter wrote:
> > GPU drivers need this in their shrinkers, to be able to throw out
> > mmap'ed buffers. Note that we also need dma_resv_lock in shrinkers,
> > but that loop is resolved by trylocking in shrinkers.
> >
> > So full hierarchy is now (ignore some of the other branches we already
> > have primed):
> >
> > mmap_read_lock -> dma_resv -> shrinkers -> i_mmap_lock_write
> >
> > I hope that's not inconsistent with anything mm or fs does, adding
> > relevant people.
> >
> Looks OK to me. The mapping_dirty_helpers run under the i_mmap_lock, but
> don't allocate any memory AFAICT.
>
> Since huge page-table-entry splitting may happen under the i_mmap_lock
> from unmap_mapping_range() it might be worth figuring out how new page
> directory pages are allocated, though.

ofc I'm not an mm expert at all, but I did try to scroll through all
i_mmap_lock_write/read callers. Found the following:

- kernel/events/uprobes.c in build_map_info:

/*
* Needs GFP_NOWAIT to avoid i_mmap_rwsem recursion through
* reclaim. This is optimistic, no harm done if it fails.
*/

- I got lost in the hugetlb.c code and couldn't convince myself it's
not allocating page directories at various levels with something else
than GFP_KERNEL.

So looks like the recursion is clearly there and known, but the
hugepage code is too complex and flying over my head.
-Daniel

>
> /Thomas
>
>
>

--
Daniel Vetter
Software Engineer, Intel Corporation