Messages in this thread | | | From | Vlastimil Babka <> | Subject | Re: [PATCH v21 15/19] mm/compaction: do page isolation first in compaction | Date | Wed, 11 Nov 2020 18:12:28 +0100 |
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On 11/5/20 9:55 AM, Alex Shi wrote: > Currently, compaction would get the lru_lock and then do page isolation > which works fine with pgdat->lru_lock, since any page isoltion would > compete for the lru_lock. If we want to change to memcg lru_lock, we > have to isolate the page before getting lru_lock, thus isoltion would > block page's memcg change which relay on page isoltion too. Then we > could safely use per memcg lru_lock later. > > The new page isolation use previous introduced TestClearPageLRU() + > pgdat lru locking which will be changed to memcg lru lock later. > > Hugh Dickins <hughd@google.com> fixed following bugs in this patch's > early version: > > Fix lots of crashes under compaction load: isolate_migratepages_block() > must clean up appropriately when rejecting a page, setting PageLRU again > if it had been cleared; and a put_page() after get_page_unless_zero() > cannot safely be done while holding locked_lruvec - it may turn out to > be the final put_page(), which will take an lruvec lock when PageLRU. > And move __isolate_lru_page_prepare back after get_page_unless_zero to > make trylock_page() safe: > trylock_page() is not safe to use at this time: its setting PG_locked > can race with the page being freed or allocated ("Bad page"), and can > also erase flags being set by one of those "sole owners" of a freshly > allocated page who use non-atomic __SetPageFlag(). > > Suggested-by: Johannes Weiner <hannes@cmpxchg.org> > Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com> > Acked-by: Hugh Dickins <hughd@google.com> > Acked-by: Johannes Weiner <hannes@cmpxchg.org> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Matthew Wilcox <willy@infradead.org> > Cc: linux-kernel@vger.kernel.org > Cc: linux-mm@kvack.org
Acked-by: Vlastimil Babka <vbabka@suse.cz>
A question below:
> @@ -979,10 +995,6 @@ static bool too_many_isolated(pg_data_t *pgdat) > goto isolate_abort; > } > > - /* Recheck PageLRU and PageCompound under lock */ > - if (!PageLRU(page)) > - goto isolate_fail; > - > /* > * Page become compound since the non-locked check, > * and it's on LRU. It can only be a THP so the order > @@ -990,16 +1002,13 @@ static bool too_many_isolated(pg_data_t *pgdat) > */ > if (unlikely(PageCompound(page) && !cc->alloc_contig)) { > low_pfn += compound_nr(page) - 1; > - goto isolate_fail; > + SetPageLRU(page); > + goto isolate_fail_put; > }
IIUC the danger here is khugepaged will collapse a THP. For that, __collapse_huge_page_isolate() has to succeed isolate_lru_page(). Under the new scheme, it shouldn't be possible, right? If that's correct, we can remove this part?
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