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

Peter Xu
2020-09-23T13:10:54Z

On Tue, Sep 22, 2020 at 08:27:35PM -0400, Peter Xu wrote:
> On Tue, Sep 22, 2020 at 04:11:16PM -0300, Jason Gunthorpe wrote:
> > 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.
>
> Now I understand your point, I think.. So I guess it's not only about
> has_pinned, but it should be a race between the fast-gup and the fork() code,
> even if has_pinned is always set.
>
> >
> > 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();
>
> Is this get_page() a must to be after ptep_set_wrprotect() explicitly? IIUC
> what we need is to order ptep_set_wrprotect() and page_maybe_dma_pinned() here.
> E.g., would a "mb()" work?
>
> Another thing is, do we need similar thing for e.g. gup_pte_range(), so that
> to guarantee ordering of try_grab_compound_head() and the pte change check?
>
> >
> > 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.
>
> Yes, iiuc do_wp_page() should be able to handle spurious write page faults like
> this already, as below:
>
> vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
> spin_lock(vmf->ptl);
> ...
> if (vmf->flags & FAULT_FLAG_WRITE) {
> if (!pte_write(entry))
> return do_wp_page(vmf);
> entry = pte_mkdirty(entry);
> }
>
> So when spin_lock() returns:
>
> - When it's a real cow (not pinned pages; we write-protected it and it keeps
> write-protected), we should do cow here as usual.
>
> - When it's a fake cow (pinned pages), the write bit should have been
> recovered before the page table lock released, and we'll skip do_wp_page()
> and retry the page fault immediately.
>
> > 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.
>
> Another question is, how about read fast-gup for pinning? Because we can't use
> the write-protect mechanism to block a read gup. I remember we've discussed
> similar things and iirc your point is "pinned pages should always be with
> WRITE". However now I still doubt it... Because I feel like read gup is still
> legal (as I mentioned previously - when device purely writes to the page and
> the processor only reads from it).
>
> >
> > 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.
>
> Tricky! I'm still thinking about some easier way but no much clue so far.
> Hopefully we'll figure out something solid soon.

Hmm, how about something like below? Would this be acceptable?

------8<--------
diff --git a/mm/gup.c b/mm/gup.c
index 2d9019bf1773..698bc2b520ac 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2136,6 +2136,18 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
struct dev_pagemap *pgmap = NULL;
int nr_start = *nr, ret = 0;
pte_t *ptep, *ptem;
+ spinlock_t *ptl = NULL;
+
+ /*
+ * More strict with FOLL_PIN, otherwise it could race with fork(). The
+ * page table lock guarantees that fork() will capture all the pinned
+ * pages when dup_mm() and do proper page copy on them.
+ */
+ if (flags & FOLL_PIN) {
+ ptl = pte_lockptr(mm, pmd);
+ if (!spin_trylock(ptl))
+ return 0;
+ }

ptem = ptep = pte_offset_map(&pmd, addr);
do {
@@ -2200,6 +2212,8 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
ret = 1;

pte_unmap:
+ if (ptl)
+ spin_unlock(ptl);
if (pgmap)
put_dev_pagemap(pgmap);
pte_unmap(ptem);
------8<--------

Both of the solution would fail some fast-gups that might have succeeded in the
past. The latter solution might even fail more (because pmd lock should be
definitely bigger than a single pte wrprotect), however afaict it's still a
very, very corner case as it's fast-gup+FOLL_PIN+lockfail (and not to mention
fast-gup should be allowed to fail).

To confirm it can fail, I also checked up that we have only one caller of
pin_user_pages_fast_only(), which is i915_gem_userptr_get_pages(). While it's:

if (mm == current->mm) {
pvec = kvmalloc_array(num_pages, sizeof(struct page *),
GFP_KERNEL |
__GFP_NORETRY |
__GFP_NOWARN);
if (pvec) {
/* defer to worker if malloc fails */
if (!i915_gem_object_is_readonly(obj))
gup_flags |= FOLL_WRITE;
pinned = pin_user_pages_fast_only(obj->userptr.ptr,
num_pages, gup_flags,
pvec);
}
}

So looks like it can fallback to something slow too even if purely unlucky. So
looks safe so far for either solution above.

--
Peter Xu