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SubjectRe: [RFC][PATCH 0/4] memcg: add support for hwpoison testing
On Tue, 1 Sep 2009 14:46:52 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> On Tue, Sep 01, 2009 at 10:32:14AM +0800, KAMEZAWA Hiroyuki wrote:
> > On Tue, 1 Sep 2009 10:25:14 +0800
> > Wu Fengguang <fengguang.wu@intel.com> wrote:
> > > > 4. I can't understand why you need this. I wonder you can get pfn via
> > > > /proc/<pid>/????. And this may insert HWPOISON to page-cache of shared
> > > > library and "unexpected" process will be poisoned.
> > >
> > > Sorry I should have explained this. It's mainly for correctness.
> > > When a user space tool queries the task PFNs in /proc/pid/pagemap and
> > > then send to /debug/hwpoison/corrupt-pfn, there is a racy window that
> > > the page could be reclaimed and allocated by some one else. It would
> > > be awkward to try to pin the pages in user space. So we need the
> > > guarantees provided by /debug/hwpoison/corrupt-filter-memcg, which
> > > will be checked inside the page lock with elevated reference count.
> > >
> >
> > memcg never holds refcnt for a page and the kernel::vmscan.c can reclaim
> > any pages under memcg whithout checking anything related to memcg.
> > *And*, your code has no "pin" code.
> > This patch sed does no jobs for your concern.
>
> We grabbed page here, which is not in the scope of this patchset:
>
> static int try_memory_failure(unsigned long pfn)
> {
> struct page *p;
> int res = -EINVAL;
>
> if (!pfn_valid(pfn))
> return res;
>
> p = pfn_to_page(pfn);
> if (!get_page_unless_zero(compound_head(p)))
> return res;
>
> lock_page_nosync(compound_head(p));
>
> if (hwpoison_filter(p))
> goto out;
>
> res = __memory_failure(pfn, 18,
> MEMORY_FAILURE_FLAG_COUNTED |
> MEMORY_FAILURE_FLAG_LOCKED);
> out:
> unlock_page(p);
> return res;
> }

Hmm. maybe off-topic but why lock_page() is necessary ?


> > I recommend you to add
> > /debug/hwpoizon/pin-pfn
> >
> > Then,
> > echo pfn > /debug/hwpoizon/pin-pfn
> > # add pfn for hwpoison debug's watch list. and elevate refcnt
> > check 'pfn' is still used.
> > echo pfn > /debug/hwpoison/corrupt-pfn
> > # check 'watch list' and make it corrupt and release refcnt.
> > or some.
>
> Looks like a good alternative. At least no more memcg dependency..
>

My point is that memcg can show 'owner' of pages but the page may
be shared with something important task _and_ if a task is migrated,
its pages' memcg information is not updated now. Then, you can kill
a task which is not in memcg.

Then, I don't recommend to use memcg. I think you'll see too much
pitfalls.

Thanks,
-Kame




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