Re: [PATCH 09/22] HWPOISON: Handle hardware poisoned pages in try_to_unmap

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Jun 17, 2009 at 10:08:13PM +0800, Minchan Kim wrote:
> On Wed, Jun 17, 2009 at 11:03 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> > On Wed, Jun 17, 2009 at 09:43:29PM +0800, Minchan Kim wrote:
> >> On Wed, Jun 17, 2009 at 10:37 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> >> > On Wed, Jun 17, 2009 at 09:27:36PM +0800, Minchan Kim wrote:
> >> >> On Wed, Jun 17, 2009 at 4:23 PM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> >> >> > On Wed, Jun 17, 2009 at 08:28:26AM +0800, Minchan Kim wrote:
> >> >> >> On Tue, 16 Jun 2009 21:49:44 +0800
> >> >> >> Wu Fengguang <fengguang.wu@intel.com> wrote:
> >> >> >>
> >> >> >> > On Tue, Jun 16, 2009 at 08:03:08AM +0800, Minchan Kim wrote:
> >> >> >> > > On Mon, 15 Jun 2009 23:26:12 +0800
> >> >> >> > > Wu Fengguang <fengguang.wu@intel.com> wrote:
> >> >> >> > >
> >> >> >> > > > On Mon, Jun 15, 2009 at 09:09:03PM +0800, Minchan Kim wrote:
> >> >> >> > > > > On Mon, Jun 15, 2009 at 11:45 AM, Wu Fengguang<fengguang.wu@intel.com> wrote:
> >> >> >> > > > > > From: Andi Kleen <ak@linux.intel.com>
> >> >> >> > > > > >
> >> >> >> > > > > > When a page has the poison bit set replace the PTE with a poison entry.
> >> >> >> > > > > > This causes the right error handling to be done later when a process runs
> >> >> >> > > > > > into it.
> >> >> >> > > > > >
> >> >> >> > > > > > Also add a new flag to not do that (needed for the memory-failure handler
> >> >> >> > > > > > later)
> >> >> >> > > > > >
> >> >> >> > > > > > Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
> >> >> >> > > > > > Signed-off-by: Andi Kleen <ak@linux.intel.com>
> >> >> >> > > > > >
> >> >> >> > > > > > ---
> >> >> >> > > > > > A include/linux/rmap.h | A  A 1 +
> >> >> >> > > > > > A mm/rmap.c A  A  A  A  A  A | A  A 9 ++++++++-
> >> >> >> > > > > > A 2 files changed, 9 insertions(+), 1 deletion(-)
> >> >> >> > > > > >
> >> >> >> > > > > > --- sound-2.6.orig/mm/rmap.c
> >> >> >> > > > > > +++ sound-2.6/mm/rmap.c
> >> >> >> > > > > > @@ -958,7 +958,14 @@ static int try_to_unmap_one(struct page
> >> >> >> > > > > > A  A  A  A /* Update high watermark before we lower rss */
> >> >> >> > > > > > A  A  A  A update_hiwater_rss(mm);
> >> >> >> > > > > >
> >> >> >> > > > > > - A  A  A  if (PageAnon(page)) {
> >> >> >> > > > > > + A  A  A  if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
> >> >> >> > > > > > + A  A  A  A  A  A  A  if (PageAnon(page))
> >> >> >> > > > > > + A  A  A  A  A  A  A  A  A  A  A  dec_mm_counter(mm, anon_rss);
> >> >> >> > > > > > + A  A  A  A  A  A  A  else if (!is_migration_entry(pte_to_swp_entry(*pte)))
> >> >> >> > > > >
> >> >> >> > > > > Isn't it straightforward to use !is_hwpoison_entry ?
> >> >> >> > > >
> >> >> >> > > > Good catch! A It looks like a redundant check: the
> >> >> >> > > > page_check_address() at the beginning of the function guarantees that
> >> >> >> > > > !is_migration_entry() or !is_migration_entry() tests will all be TRUE.
> >> >> >> > > > So let's do this?
> >> >> >> > > It seems you expand my sight :)
> >> >> >> > >
> >> >> >> > > I don't know migration well.
> >> >> >> > > How page_check_address guarantee it's not migration entry ?
> >> >> >> >
> >> >> >> > page_check_address() calls pte_present() which returns the
> >> >> >> > (_PAGE_PRESENT | _PAGE_PROTNONE) bits. While x86-64 defines
> >> >> >> >
> >> >> >> > #define __swp_entry(type, offset) A  A  A  ((swp_entry_t) { \
> >> >> >> > A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A ((type) << (_PAGE_BIT_PRESENT + 1)) \
> >> >> >> > A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A | ((offset) << SWP_OFFSET_SHIFT) })
> >> >> >> >
> >> >> >> > where SWP_OFFSET_SHIFT is defined to the bigger one of
> >> >> >> > max(_PAGE_BIT_PROTNONE + 1, _PAGE_BIT_FILE + 1) = max(8+1, 6+1) = 9.
> >> >> >> >
> >> >> >> > So __swp_entry(type, offset) := (type << 1) | (offset << 9)
> >> >> >> >
> >> >> >> > We know that the swap type is 5 bits. So the bit 0 _PAGE_PRESENT and bit 8
> >> >> >> > _PAGE_PROTNONE will all be zero for swap entries.
> >> >> >> >
> >> >> >>
> >> >> >> Thanks for kind explanation :)
> >> >> >
> >> >> > You are welcome~
> >> >> >
> >> >> >> >
> >> >> >> > > In addtion, If the page is poison while we are going to
> >> >> >> > > migration((PAGE_MIGRATION && migration) == TRUE), we should decrease
> >> >> >> > > file_rss ?
> >> >> >> >
> >> >> >> > It will die on trying to migrate the poisoned page so we don't care
> >> >> >> > the accounting. But normally the poisoned page shall already be
> >> >> >>
> >> >> >>
> >> >> >> Okay. then, how about this ?
> >> >> >> We should not increase file_rss on trying to migrate the poisoned page
> >> >> >>
> >> >> >> - A  A  A  A  A  A  A  else if (!is_migration_entry(pte_to_swp_entry(*pte)))
> >> >> >> + A  A  A  A  A  A  A  else if (!(PAGE_MIGRATION && migration))
> >> >> >
> >> >> > This is good if we are going to stop the hwpoison page from being
> >> >> > consumed by move_to_new_page(), but I highly doubt we'll ever add
> >> >> > PageHWPoison() checks into the migration code.
> >> >> >
> >> >> > Because this race window is small enough:
> >> >> >
> >> >> > A  A  A  A TestSetPageHWPoison(p);
> >> >> > A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  lock_page(page);
> >> >> > A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  A  try_to_unmap(page, TTU_MIGRATION|...);
> >> >> > A  A  A  A lock_page_nosync(p);
> >> >> >
> >> >> > such small race windows can be found all over the kernel, it's just
> >> >> > insane to try to fix any of them.
> >> >>
> >> >> Sorry for too late response.
> >> >>
> >> >> I see your point.
> >> >> My opinion is that at least we must be notified when such situation happen.
> >> >> So I think it would be better to add some warning to fix up it when it
> >> >> happen even thought A it is small race window.
> >> >
> >> > Notification is also pointless here: we'll die hard on
> >> > accessing/consuming the poisoned page anyway :(
> >>
> >> My intention wasn't to recover it.
> >
> > Yes, that's not the point.
> >
> >> It just add something like WARN_ON.
> >> You said it is small window enough. but I think it can happen more
> >> hight probability in migration-workload.(At a moment, I don't know
> >> what kinds of app)
> >> For such case, If we can hear reporting of warning, at that time we
> >> can consider migration handling for HWPoison.
> >
> > The point is, any page can go corrupted any time. We don't need to add
> > 1000 PageHWPoison() tests in the kernel like this. We don't aim for
> > 100% protection, that's impossible. I'd be very contented if ever it
> > can reach 80% coverage :)
> 
> Okay.
> If it is your goal, I also think migration portion of all is very small.
> Thanks for kind reply for my boring discussion.
> 
> Reviewed-by : Minchan Kim <minchan.kim@gmail.com>

Thank you, I'll add comments to clearly state that goal and its rational :)

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