Re: [PATCH] mm: page_alloc: store updated page migratetype to avoid misusing stale value

From: Vlastimil Babka
Date: Wed Nov 19 2014 - 16:28:36 EST

On 11/17/2014 11:40 AM, Weijie Yang wrote:
> The commit ad53f92e(fix incorrect isolation behavior by rechecking migratetype)
> patch series describe the race between page isolation and free path, and try to
> fix the freepage account issues.
> However, there is still a little issue: freed page could have stale migratetype
> in the free_list. This would cause some bad behavior if we misuse this stale
> value later.
> Such as: in __test_page_isolated_in_pageblock() we check the buddy page, if the
> page's stale migratetype is not MIGRATE_ISOLATE, which will cause unnecessary
> page move action.


are there other places than __test_page_isolated_in_pageblock(), where
freepage_migratetype matters? You make it sound like it's just an example, but I
doubt there is any other. All other callers of get_freepage_migratetype() are
querying pages on the pcplists, not the buddy lists. There it serves as a cached
value for migratetype so it doesn't have to be read again when freeing from
pcplists to budy list.

Seems to me that __test_page_isolated_in_pageblock() was an exception that tried
to rely on freepage_migratetype being valid even after the page has moved from
pcplist to buddy list, but this assumption was always broken.
Sure, if all the pages in isolated pageblock are catched by move_freepages()
during isolation, then the freetype is correct, but that doesn't always happen
due to parallel activity (and that's the core problem that Joonsoo's series
dealt with).

So, in this patch you try to make sure that freepage_migratetype will be correct
after page got to buddy list via __free_one_page(). But I don't think that
covers all situations. Look at expand(), which puts potentially numerous
splitted free pages on free_list, and doesn't set freepage_migratetype. Sure,
this ommision doesn't affect __test_page_isolated_in_pageblock(), as expand() is
called during allocation, which won't touch ISOLATE pageblocks, and free pages
created by expand() *before* setting ISOLATE are processed by move_freepages().

So my point is, you are maybe fixing just the case of
__test_page_isolated_in_pageblock() (but not completely I think, see below) by
adding extra operation to __free_one_page() which is hot path. And all for a
WARN_ON_ONCE. That doesn't seem like a good tradeoff. And to do it consistently,
you would need to add the operation also to expand(), another hotpath. So that's
a NAK from me.

I would uggest you throw the __test_page_isolated_in_pageblock() function away
completely. Or just rework it to check for PageBuddy() and hwpoison stuff - the
migratetype checks make no sense to me. Or if you insist that this is needed for
debugging further possible races in page isolation, then please hide the
necessary bits in hot paths being a debugging config option.

If you agree, we can even throw away the set_freepage_migratetype() calls from
move_freepages() - there's no point to them anymore.

> This patch store the page's updated migratetype after free the page to the
> free_list to avoid subsequent misusing stale value, and use a WARN_ON_ONCE
> to catch a potential undetected race between isolatation and free path.
> Signed-off-by: Weijie Yang <weijie.yang@xxxxxxxxxxx>
> ---
> mm/page_alloc.c | 1 +
> mm/page_isolation.c | 17 +++++------------
> 2 files changed, 6 insertions(+), 12 deletions(-)
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 616a2c9..177fca0 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -622,6 +622,7 @@ static inline void __free_one_page(struct page *page,

See here at this point, the function has this code:
goto out;

You are missing this list_add_tail() path with your patch.

> }
> list_add(&page->lru, &zone->free_area[order].free_list[migratetype]);
> + set_freepage_migratetype(page, migratetype);
> out:
> zone->free_area[order].nr_free++;
> }
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index c8778f7..0618071 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -223,19 +223,12 @@ __test_page_isolated_in_pageblock(unsigned long pfn, unsigned long end_pfn,
> page = pfn_to_page(pfn);
> if (PageBuddy(page)) {
> /*
> - * If race between isolatation and allocation happens,
> - * some free pages could be in MIGRATE_MOVABLE list
> - * although pageblock's migratation type of the page
> - * is MIGRATE_ISOLATE. Catch it and move the page into
> + * Use a WARN_ON_ONCE to catch a potential undetected
> + * race between isolatation and free pages, even if
> + * we try to avoid this issue.
> */
> - if (get_freepage_migratetype(page) != MIGRATE_ISOLATE) {
> - struct page *end_page;
> -
> - end_page = page + (1 << page_order(page)) - 1;
> - move_freepages(page_zone(page), page, end_page,
> - }
> + WARN_ON_ONCE(get_freepage_migratetype(page) !=

So yeah as I said, all the trouble and inconsistency for a WARN_ON_ONCE doesn't
seem worth it.

> pfn += 1 << page_order(page);
> }

BTW, here the function continues like:

else if (page_count(page) == 0 &&
get_freepage_migratetype(page) == MIGRATE_ISOLATE)
pfn += 1;

I believe this code tries to check for pages on pcplists? But isn't it bogus? At
least currently, page is never added to pcplist with MIGRATE_ISOLATE
freepage_migratetype - it goes straight to buddy lists.
Also, why count pages on pcplists as successfully isolated?
isolate_freepages_range() will fail on them.

> else if (page_count(page) == 0 &&

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