Re: [PATCH 0/3] Fix migration races in rmap_walk() V2

[KAMEZAWA Hiroyuki]

On Wed, 28 Apr 2010 00:32:42 +0200
Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:

> On Tue, Apr 27, 2010 at 05:27:36PM -0500, Christoph Lameter wrote:
> > Can we simply wait like in the fault path?
> 
> There is no bug there, no need to wait either. I already audited it
> before, and I didn't see any bug. Unless you can show a bug with CPU A
> running the rmap_walk on process1 before process2, there is no bug to
> fix there.
> 
I think there is no bug, either. But that safety is fragile.


> > 
> > > Patch 3 notes that while a VMA is moved under the anon_vma lock, the page
> > > 	tables are not similarly protected. Where migration PTEs are
> > > 	encountered, they are cleaned up.
> > 
> > This means they are copied / moved etc and "cleaned" up in a state when
> > the page was unlocked. Migration entries are not supposed to exist when
> > a page is not locked.
> 
> patch 3 is real, and the first thought I had was to lock down the page
> before running vma_adjust and unlock after move_page_tables. But these
> are virtual addresses. Maybe there's a simpler way to keep migration
> away while we run those two operations.
> 

Doing some check in move_ptes() after vma_adjust() is not safe.
IOW, when vma's information and information in page-table is incosistent...objrmap
is broken and migartion will cause panic.

Then...I think there are 2 ways.
  1. use seqcounter in "mm_struct" as previous patch and lock it at mremap.
or
  2. get_user_pages_fast() when do remap.

Thanks,
-Kame







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