To avoid a race between rmap walk and mremap, mremap does take_rmap_locks(). The lock was taken to ensure that rmap walk don't miss a page table entry due to PTE moves via move_pagetables(). The kernel does further optimization of this lock such that if we are going to find the newly added vma after the old vma, the rmap lock is not taken. This is because rmap walk would find the vmas in the same order and if we don't find the page table attached to older vma we would find it with the new vma which we would iterate later.
As explained in commit eb66ae030829 ("mremap: properly flush TLB before releasing the page") mremap is special in that it doesn't take ownership of the page. The optimized version for PUD/PMD aligned mremap also doesn't hold the ptl lock. This can result in stale TLB entries as show below.
This patch updates the rmap locking requirement in mremap to handle the race condition explained below with optimized mremap::
Optmized PMD move
CPU 1 CPU 2 CPU 3
mremap(old_addr, new_addr) page_shrinker/try_to_unmap_one
mmap_write_lock_killable()
addr = old_addr lock(pte_ptl) lock(pmd_ptl) pmd = *old_pmd pmd_clear(old_pmd) flush_tlb_range(old_addr)
*new_pmd = pmd *new_addr = 10; and fills TLB with new addr and old pfn
unlock(pmd_ptl) ptep_clear_flush() old pfn is free. Stale TLB entry
Optimized PUD move also suffers from a similar race. Both the above race condition can be fixed if we force mremap path to take rmap lock.
Cc: stable@vger.kernel.org Fixes: 2c91bd4a4e2e ("mm: speed up mremap by 20x on large regions") Fixes: c49dd3401802 ("mm: speedup mremap on 1GB or larger regions") Link: https://lore.kernel.org/linux-mm/CAHk-=wgXVR04eBNtxQfevontWnP6FDm+oj5vauQXP3... Acked-by: Hugh Dickins hughd@google.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Signed-off-by: Aneesh Kumar K.V aneesh.kumar@linux.ibm.com --- mm/mremap.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/mm/mremap.c b/mm/mremap.c index 72fa0491681e..c3cad539a7aa 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -503,7 +503,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, } else if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD) && extent == PUD_SIZE) {
if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr, - old_pud, new_pud, need_rmap_locks)) + old_pud, new_pud, true)) continue; }
@@ -530,7 +530,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, * moving at the PMD level if possible. */ if (move_pgt_entry(NORMAL_PMD, vma, old_addr, new_addr, - old_pmd, new_pmd, need_rmap_locks)) + old_pmd, new_pmd, true)) continue; }