Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

Jason Gunthorpe
2020-09-22T19:11:29Z

On Tue, Sep 22, 2020 at 01:54:15PM -0400, Peter Xu wrote:
> diff --git a/mm/memory.c b/mm/memory.c
> index 8f3521be80ca..6591f3f33299 100644
> +++ b/mm/memory.c
> @@ -888,8 +888,8 @@ copy_one_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> * Because we'll need to release the locks before doing cow,
> * pass this work to upper layer.
> */
> - if (READ_ONCE(src_mm->has_pinned) && wp &&
> - page_maybe_dma_pinned(page)) {
> + if (wp && page_maybe_dma_pinned(page) &&
> + READ_ONCE(src_mm->has_pinned)) {
> /* We've got the page already; we're safe */
> data->cow_old_page = page;
> data->cow_oldpte = *src_pte;
>
> I can also add some more comment to emphasize this.

It is not just that, but the ptep_set_wrprotect() has to be done
earlier.

Otherwise it races like:

pin_user_pages_fast() fork()
atomic_set(has_pinned, 1);
[..]
atomic_read(page->_refcount) //false
// skipped atomic_read(has_pinned)
atomic_add(page->_refcount)
ordered check write protect()
ordered set write protect()

And now have a write protect on a DMA pinned page, which is the
invarient we are trying to create.

The best algorithm I've thought of is something like:

pte_map_lock()
if (page) {
if (wp) {
ptep_set_wrprotect()
/* Order with try_grab_compound_head(), either we see
* page_maybe_dma_pinned(), or they see the wrprotect */
get_page();

if (page_maybe_dma_pinned() && READ_ONCE(src_mm->has_pinned)) {
put_page();
ptep_clear_wrprotect()

// do copy
return
}
} else {
get_page();
}
page_dup_rmap()
pte_unmap_lock()

Then the do_wp_page() path would have to detect that the page is not
write protected under the pte lock inside the fault handler and just
do nothing. Ie the set/clear could be visible to the CPU and trigger a
spurious fault, but never trigger a COW.

Thus 'wp' becomes a 'lock' that prevents GUP from returning this page.

Very tricky, deserves a huge comment near the ptep_clear_wrprotect()

Consider the above algorithm beside the gup_fast() algorithm:

if (!pte_access_permitted(pte, flags & FOLL_WRITE))
goto pte_unmap;
[..]
head = try_grab_compound_head(page, 1, flags);
if (!head)
goto pte_unmap;
if (unlikely(pte_val(pte) != pte_val(*ptep))) {
put_compound_head(head, 1, flags);
goto pte_unmap;

That last *ptep will check that the WP is not set after making
page_maybe_dma_pinned() true.

It still looks reasonable, the extra work is still just the additional
atomic in page_maybe_dma_pinned(), just everything else has to be very
carefully sequenced due to unlocked page table accessors.

> I think the WRITE_ONCE/READ_ONCE can actually be kept, because atomic ops
> should contain proper memory barriers already so the memory access orders
> should be guaranteed

I always have to carefully check ORDERING in
Documentation/atomic_t.txt when asking those questions..

It seems very subtle to me, but yes, try_grab_compound_head() and
page_maybe_dma_pinned() are already paired ordering barriers, so both
the pte_val() on the GUP side and the READ_ONCE(has_pinned) look OK.

Jason