Re: [RFC][PATCH v2 1/5] mm: Avoid unmapping pinned pages
From: Peter Zijlstra <peterz@infradead.org>
Date: 2022-01-21 07:52:33
Also in:
linux-mm, lkml
On Thu, Jan 20, 2022 at 07:25:08PM +0100, David Hildenbrand wrote:
On 20.01.22 16:55, Peter Zijlstra wrote:quoted
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...