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SubjectRe: [RFC][PATCH 0/4] memcg: add support for hwpoison testing
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;
}

> 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..

Cheers,
Fengguang


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