Re: [PATCH 22/23] userfaultfd: avoid mmap_sem read recursion in mcopy_atomic
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2015-05-22 20:18:27
Also in:
kvm, linux-mm, lkml, qemu-devel
On Thu, 14 May 2015 19:31:19 +0200 Andrea Arcangeli [off-list ref] wrote:
quoted hunk ↗ jump to hunk
If the rwsem starves writers it wasn't strictly a bug but lockdep doesn't like it and this avoids depending on lowlevel implementation details of the lock. ...@@ -229,13 +246,33 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm, if (!zeropage) err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma, - dst_addr, src_addr); + dst_addr, src_addr, &page); else err = mfill_zeropage_pte(dst_mm, dst_pmd, dst_vma, dst_addr); cond_resched(); + if (unlikely(err == -EFAULT)) { + void *page_kaddr; + + BUILD_BUG_ON(zeropage);
I'm not sure what this is trying to do. BUILD_BUG_ON(local_variable)? It goes bang in my build. I'll just delete it.
+ up_read(&dst_mm->mmap_sem);
+ BUG_ON(!page);
+
+ page_kaddr = kmap(page);
+ err = copy_from_user(page_kaddr,
+ (const void __user *) src_addr,
+ PAGE_SIZE);
+ kunmap(page);
+ if (unlikely(err)) {
+ err = -EFAULT;
+ goto out;
+ }
+ goto retry;
+ } else
+ BUG_ON(page);
+