Re: [RFC][PATCH v2 1/5] mm: Avoid unmapping pinned pages

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 20, 2022 at 07:25:08PM +0100, David Hildenbrand wrote:
> On 20.01.22 16:55, Peter Zijlstra wrote:
> > Add a guarantee for Anon pages that pin_user_page*() ensures the
> > user-mapping of these pages stay preserved. In order to ensure this
> > all rmap users have been audited:
> > 
> >  vmscan:	already fails eviction due to page_maybe_dma_pinned()
> > 
> >  migrate:	migration will fail on pinned pages due to
> > 		expected_page_refs() not matching, however that is
> > 		*after* try_to_migrate() has already destroyed the
> > 		user mapping of these pages. Add an early exit for
> > 		this case.
> > 
> >  numa-balance:	as per the above, pinned pages cannot be migrated,
> > 		however numa balancing scanning will happily PROT_NONE
> > 		them to get usage information on these pages. Avoid
> > 		this for pinned pages.
> 
> page_maybe_dma_pinned() can race with GUP-fast without
> mm->write_protect_seq. This is a real problem for vmscan() with
> concurrent GUP-fast as it can result in R/O mappings of pinned pages and
> GUP will lose synchronicity to the page table on write faults due to
> wrong COW.

Urgh, so yeah, that might be a problem. Follow up code uses it like
this:

+/*
+ * Pinning a page inhibits rmap based unmap for Anon pages. Doing a load
+ * through the user mapping ensures the user mapping exists.
+ */
+#define umcg_pin_and_load(_self, _pagep, _member)                              \
+({                                                                             \
+       __label__ __out;                                                        \
+       int __ret = -EFAULT;                                                    \
+                                                                               \
+       if (pin_user_pages_fast((unsigned long)(_self), 1, 0, &(_pagep)) != 1)  \
+               goto __out;                                                     \
+                                                                               \
+       if (!PageAnon(_pagep) ||                                                \
+           get_user(_member, &(_self)->_member)) {                             \
+               unpin_user_page(_pagep);                                        \
+               goto __out;                                                     \
+       }                                                                       \
+       __ret = 0;                                                              \
+__out: __ret;                                                                  \
+})

And after that hard assumes (on the penalty of SIGKILL) that direct user
access works. Specifically it does RmW ops on it. So I suppose I'd
better upgrade that load to a RmW at the very least.

But is that sufficient? Let me go find that race you mention...