mremap: properly flush TLB before releasing the page

Jann Horn points out that our TLB flushing was subtly wrong for the
mremap() case.  What makes mremap() special is that we don't follow the
usual "add page to list of pages to be freed, then flush tlb, and then
free pages".  No, mremap() obviously just _moves_ the page from one page
table location to another.

That matters, because mremap() thus doesn't directly control the
lifetime of the moved page with a freelist: instead, the lifetime of the
page is controlled by the page table locking, that serializes access to
the entry.

As a result, we need to flush the TLB not just before releasing the lock
for the source location (to avoid any concurrent accesses to the entry),
but also before we release the destination page table lock (to avoid the
TLB being flushed after somebody else has already done something to that
page).

This also makes the whole "need_flush" logic unnecessary, since we now
always end up flushing the TLB for every valid entry.

Reported-and-tested-by: Jann Horn <jannh@google.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 4 insertions, 6 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 58269f8ba7c4..deed97fba979 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1780,7 +1780,7 @@ static pmd_t move_soft_dirty_pmd(pmd_t pmd)
 
 bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
 		  unsigned long new_addr, unsigned long old_end,
-		  pmd_t *old_pmd, pmd_t *new_pmd, bool *need_flush)
+		  pmd_t *old_pmd, pmd_t *new_pmd)
 {
 	spinlock_t *old_ptl, *new_ptl;
 	pmd_t pmd;
@@ -1811,7 +1811,7 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
 		if (new_ptl != old_ptl)
 			spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
 		pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
-		if (pmd_present(pmd) && pmd_dirty(pmd))
+		if (pmd_present(pmd))
 			force_flush = true;
 		VM_BUG_ON(!pmd_none(*new_pmd));
 
@@ -1822,12 +1822,10 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
 		}
 		pmd = move_soft_dirty_pmd(pmd);
 		set_pmd_at(mm, new_addr, new_pmd, pmd);
-		if (new_ptl != old_ptl)
-			spin_unlock(new_ptl);
 		if (force_flush)
 			flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
-		else
-			*need_flush = true;
+		if (new_ptl != old_ptl)
+			spin_unlock(new_ptl);
 		spin_unlock(old_ptl);
 		return true;
 	}