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

[Minchan Kim]

On Wed, Jun 17, 2009 at 4:23 PM, Wu Fengguang<fengguang.wu@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> wrote:
>> > > > > > From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> > > > > >
>> > > > > > 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@xxxxxxxxx>
>> > > > > > Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>> > > > > >
>> > > > > > ---
>> > > > > > Âinclude/linux/rmap.h | Â Â1 +
>> > > > > > Âmm/rmap.c      Â|  Â9 ++++++++-
>> > > > > > Â2 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
>> > > > > > Â Â Â Â/* Update high watermark before we lower rss */
>> > > > > > Â Â Â Âupdate_hiwater_rss(mm);
>> > > > > >
>> > > > > > - Â Â Â if (PageAnon(page)) {
>> > > > > > + Â Â Â if (PageHWPoison(page) && !(flags & TTU_IGNORE_HWPOISON)) {
>> > > > > > + Â Â Â Â Â Â Â if (PageAnon(page))
>> > > > > > + Â Â Â Â Â Â Â Â Â Â Â dec_mm_counter(mm, anon_rss);
>> > > > > > + Â Â Â Â Â Â Â else if (!is_migration_entry(pte_to_swp_entry(*pte)))
>> > > > >
>> > > > > Isn't it straightforward to use !is_hwpoison_entry ?
>> > > >
>> > > > Good catch! ÂIt 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) Â Â Â ((swp_entry_t) { \
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â((type) << (_PAGE_BIT_PRESENT + 1)) \
>> > Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â| ((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
>>
>> - Â Â Â Â Â Â Â else if (!is_migration_entry(pte_to_swp_entry(*pte)))
>> + Â Â Â Â Â Â Â 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:
>
> Â Â Â ÂTestSetPageHWPoison(p);
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â lock_page(page);
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â try_to_unmap(page, TTU_MIGRATION|...);
> Â Â Â Âlock_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  it is small race window.

-- 
Kinds regards,
Minchan Kim
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