On 2021/5/6 15:30, HORIGUCHI NAOYA(堀口 直也) wrote:
On Thu, May 06, 2021 at 12:01:34PM +0800, Ding Hui wrote:
On 2021/5/6 10:49, HORIGUCHI NAOYA(堀口 直也) wrote:
On Wed, Apr 28, 2021 at 04:54:59PM +0200, David Hildenbrand wrote:
On 21.04.21 04:04, Ding Hui wrote:
Recently we found there is a lot MemFree left in /proc/meminfo after
do a lot of pages soft offline.
I think it's incorrect since NR_FREE_PAGES should not contain HWPoison pages.
After take_page_off_buddy, the page is no longer belong to buddy
allocator, and will not be used any more, but we maybe missed accounting
NR_FREE_PAGES in this situation.
Signed-off-by: Ding Hui <dinghui@xxxxxxxxxxxxxx>
---
mm/page_alloc.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cfc72873961d..8d65b62784d8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -8947,6 +8947,7 @@ bool take_page_off_buddy(struct page *page)
del_page_from_free_list(page_head, zone, page_order);
break_down_buddy_pages(zone, page_head, page, 0,
page_order, migratetype);
+ __mod_zone_page_state(zone, NR_FREE_PAGES, -1);
ret = true;
break;
}
Should this use __mod_zone_freepage_state() instead?
Yes, __mod_zone_freepage_state() looks better to me.
And I think that maybe an additional __mod_zone_freepage_state() in
unpoison_memory() is necessary to cancel the decrement. I thought of the
following, but it doesn't build because get_pfnblock_migratetype() is
available only in mm/page_alloc.c, so you might want to add a small exported
routine in mm/page_alloc.c and let it called from unpoison_memory().
@@ -1899,8 +1899,12 @@ int unpoison_memory(unsigned long pfn)
}
if (!get_hwpoison_page(p, flags, 0)) {
- if (TestClearPageHWPoison(p))
+ if (TestClearPageHWPoison(p)) {
+ int migratetype = get_pfnblock_migratetype(p, pfn);
+
num_poisoned_pages_dec();
+ __mod_zone_freepage_state(page_zone(p), 1, migratetype);
+ }
unpoison_pr_info("Unpoison: Software-unpoisoned free page %#lx\n",
pfn, &unpoison_rs);
return 0;
I think there is another problem:
In normal case, we keep the last refcount of the hwpoison page, so
get_hwpoison_page should return 1. The NR_FREE_PAGES will be adjusted when
call put_page.
I think that take_page_off_buddy() should not be called for this case
(the error page have remaining refcount). So it seems that no need to
update NR_FREE_PAGES ?
Yes, take_page_off_buddy() only used for free pages, but we will call
page_ref_inc() after that, on the other hand for in used pages, we
increased the refcount by get_any_page(), so in both cases, the
hwpoisoned pages have refcount great than zero.
I think there is no need to update NR_FREE_PAGES explicitly in
unpoison_memory(), the put_page() will help us to update NR_FREE_PAGES
and put the page back to buddy.
At race condition, we maybe leak the page because we does not put it back to
buddy in unpoison_memory, however the HWPoison flag, num_poisoned_pages,
NR_FREE_PAGES is adjusted correctly.
CPU0 CPU1
soft_offline_page
soft_offline_free_page
page_handle_poison
take_page_off_buddy
SetPageHWPoison
unpoison_memory
if (!get_hwpoison_page(p))
TestClearPageHWPoison
num_poisoned_pages_dec
__mod_zone_freepage_state
return 0
/* miss put the page back to buddy */
page_ref_inc
num_poisoned_pages_inc
Thanks for checking this, unpoison_memory() is racy. Recently we are suggesting
to introduce mf_mutex by [1]. Although this patch is not merged to mainline yet,
but it could be used to prevent the above race too.
[1] https://lore.kernel.org/linux-mm/20210427062953.2080293-2-nao.horiguchi@xxxxxxxxx/
I'll look forward to it, thanks.
How about do nothing and return -EBUSY (so the caller can retry) if unpoison
a zero refcount page , or return 0 like 230ac719c500 ("mm/hwpoison: don't
try to unpoison containment-failed pages") does ?
@@ -1736,11 +1736,9 @@ int unpoison_memory(unsigned long pfn)
}
if (!get_hwpoison_page(p, flags, 0)) {
- if (TestClearPageHWPoison(p))
- num_poisoned_pages_dec();
- unpoison_pr_info("Unpoison: Software-unpoisoned free page %#lx\n",
+ unpoison_pr_info("Unpoison: Software-unpoisoned zero refcount page
%#lx\n",
pfn, &unpoison_rs);
- return 0;
+ return -EBUSY;
Currently unpoison_memory() does not work as reverse operation of take_page_off_buddy()
(it's simply broken), so implementing it at one time would be better.
I'll take time to fix unpoison_memory().
Thanks for your work.
Actually, I'm not sure about the exactly meaning of "broken", it seems
that the basic function of unpoison_memory() is ok if not considered the
race conditions.
--
Thanks,
- Ding Hui
