On 10/23/20 5:19 PM, Jason Gunthorpe wrote:
Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during
fork() for ptes") pages under a FOLL_PIN will not be write protected
during COW for fork. This means that pages returned from
pin_user_pages(FOLL_WRITE) should not become write protected while the pin
is active.
However, there is a small race where get_user_pages_fast(FOLL_PIN) can
establish a FOLL_PIN at the same time copy_present_page() is write
protecting it:
CPU 0 CPU 1
get_user_pages_fast()
internal_get_user_pages_fast()
copy_page_range()
pte_alloc_map_lock()
copy_present_page()
atomic_read(has_pinned) == 0
page_maybe_dma_pinned() == false
atomic_set(has_pinned, 1);
gup_pgd_range()
gup_pte_range()
pte_t pte = gup_get_pte(ptep)
pte_access_permitted(pte)
try_grab_compound_head()
pte = maybe_mkwrite()
set_pte_at();
pte_unmap_unlock()
// GUP now returns with a write protected page
The first attempt to resolve this by using the write protect caused
problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid
early COW write protect games during fork()")
Instead wrap copy_p4d_range() with the write side of something like a
seqcount and check the read side around gup_pgd_range(). If there is a
collision then get_user_pages_fast() fails and falls back to slow GUP.
Slow GUP is safe against this race because copy_page_range() is only
called while holding the write side of the mmap_lock on the src mm_struct.
Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()")
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@xxxxxxxxxxxxxx
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
---
include/linux/mm_types.h | 6 ++++++
kernel/fork.c | 1 +
mm/gup.c | 19 +++++++++++++++++++
mm/memory.c | 16 +++++++++++++++-
4 files changed, 41 insertions(+), 1 deletion(-)
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5a9238f6caad97..8c7c9de476c4f8 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -446,6 +446,12 @@ struct mm_struct {
*/
atomic_t has_pinned;
+ /**
+ * @write_protecet_seq: Odd when any thread is write
typo, make that: @write_protect_seq
Given that this is an open-coded seqlock-like thing, I think it deserves
quite a bit more comment. Maybe something like the comment that you already
drafted for memory.c.
But as I mentioned in the reply to the cover letter, what I'd really prefer
is just using the raw seqlock API. I really-really-really think it's better
for the situation, than doing it this way.
+ * protecting pages in this mm, for instance during fork().
+ */
+ unsigned long write_protect_seq;
+
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* PTE page table pages */
#endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 32083db7a2a23e..342243f621c742 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1007,6 +1007,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm->vmacache_seqnum = 0;
atomic_set(&mm->mm_users, 1);
atomic_set(&mm->mm_count, 1);
+ mm->write_protect_seq = 0;
mmap_init_lock(mm);
INIT_LIST_HEAD(&mm->mmlist);
mm->core_state = NULL;
diff --git a/mm/gup.c b/mm/gup.c
index ecbe1639ea2af7..2c1a1e0555479e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2677,12 +2677,19 @@ static unsigned int lockless_pages_from_mm(unsigned long addr,
struct page **pages)
{
unsigned long flags;
+ unsigned long seq;
int nr_pinned = 0;
if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
!gup_fast_permitted(addr, end))
return 0;
+ if (gup_flags & FOLL_PIN) {
+ seq = smp_load_acquire(¤t->mm->write_protect_seq);
+ if (seq & 1)
+ return 0;
+ }
+
/*
* Disable interrupts. The nested form is used, in order to allow full,
* general purpose use of this routine.
@@ -2697,6 +2704,18 @@ static unsigned int lockless_pages_from_mm(unsigned long addr,
local_irq_save(flags);
gup_pgd_range(addr, end, gup_flags, pages, &nr_pinned);
local_irq_restore(flags);
+
+ /*
+ * When pinning pages for DMA there could be a concurrent write protect
+ * from fork() via copy_page_range(), in this case always fail fast GUP.
+ */
+ if (gup_flags & FOLL_PIN) {
+ smp_rmb();
+ if (READ_ONCE(current->mm->write_protect_seq) != seq) {
So, above we use smp_load_acquire() to read this, but here we use use smp_rmb()
plus READ_ONCE(). OK, I am in over my head, even with memory-barriers.txt. :)
Hopefully someone with skill in this area can help review that.
+ unpin_user_pages(pages, nr_pinned);
+ return 0;
+ }
+ }
return nr_pinned;
}
diff --git a/mm/memory.c b/mm/memory.c
index c48f8df6e50268..e2f959cce8563d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1171,6 +1171,17 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
0, src_vma, src_mm, addr, end);
mmu_notifier_invalidate_range_start(&range);
+ /*
+ * This is like a seqcount where the mmap_lock provides
+ * serialization for the write side. However, unlike seqcount
+ * the read side falls back to obtaining the mmap_lock rather
+ * than spinning. For this reason none of the preempt related
+ * machinery in seqcount is desired here.
+ */
+ mmap_assert_write_locked(src_mm);
+ WRITE_ONCE(src_mm->write_protect_seq,
+ src_mm->write_protect_seq + 1);
+ smp_wmb();
Even if you don't take the "use the raw seqlock API" advice, it seems like these
operations could be wrapped up in a function call, yes?
thanks,
--
John Hubbard
NVIDIA
}
ret = 0;
@@ -1187,8 +1198,11 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
}
} while (dst_pgd++, src_pgd++, addr = next, addr != end);
- if (is_cow)
+ if (is_cow) {
+ smp_store_release(&src_mm->write_protect_seq,
+ src_mm->write_protect_seq + 1);
mmu_notifier_invalidate_range_end(&range);
+ }
return ret;
}
