On 06.04.23 19:29, Peter Xu wrote:
Hi, Lokesh,
Sorry for a late reply. Copy Blake Caldwell and Mike too.
On Thu, Feb 16, 2023 at 02:27:11PM -0800, Lokesh Gidra wrote:
I) SUMMARY:
Requesting comments on a new feature which remaps pages from one
private anonymous mapping to another, without altering the vmas
involved. Two alternatives exist but both have drawbacks:
1. userfaultfd ioctls allocate new pages, copy data and free the old
ones even when updates could be done in-place;
2. mremap results in vma splitting in most of the cases due to 'pgoff' mismatch.
Personally it was always a mistery to me on how vm_pgoff works with
anonymous vmas and why it needs to be setup with vm_start >> PAGE_SHIFT.
Just now I tried to apply below oneliner change:
@@ -1369,7 +1369,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
/*
* Set pgoff according to addr for anon_vma.
*/
- pgoff = addr >> PAGE_SHIFT;
+ pgoff = 0;
break;
default:
return -EINVAL;
The kernel even boots without a major problem so far..
I think it's for RMAP purposes.
Take a look at linear_page_index() and how it's, for example, used in
ksm_might_need_to_copy() alongside page->index.
--
Thanks,
David / dhildenb
