From: Vlastimil Babka <vbabka@suse.cz>
To: Hugh Dickins <hughd@google.com>, Baoquan He <bhe@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>,
David Hildenbrand <david@redhat.com>,
linux-mm@kvack.org, pifang@redhat.com,
linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
aarcange@redhat.com, Mel Gorman <mgorman@suse.de>
Subject: Re: Memory hotplug softlock issue
Date: Tue, 20 Nov 2018 14:38:23 +0100 [thread overview]
Message-ID: <3f1a82a8-f2aa-ac5e-e6a8-057256162321@suse.cz> (raw)
In-Reply-To: <alpine.LSU.2.11.1811192127130.2848@eggly.anvils>
On 11/20/18 6:44 AM, Hugh Dickins wrote:
> [PATCH] mm: put_and_wait_on_page_locked() while page is migrated
>
> We have all assumed that it is essential to hold a page reference while
> waiting on a page lock: partly to guarantee that there is still a struct
> page when MEMORY_HOTREMOVE is configured, but also to protect against
> reuse of the struct page going to someone who then holds the page locked
> indefinitely, when the waiter can reasonably expect timely unlocking.
>
> But in fact, so long as wait_on_page_bit_common() does the put_page(),
> and is careful not to rely on struct page contents thereafter, there is
> no need to hold a reference to the page while waiting on it. That does
So there's still a moment where refcount is elevated, but hopefully
short enough, right? Let's see if it survives Baoquan's stress testing.
> mean that this case cannot go back through the loop: but that's fine for
> the page migration case, and even if used more widely, is limited by the
> "Stop walking if it's locked" optimization in wake_page_function().
>
> Add interface put_and_wait_on_page_locked() to do this, using negative
> value of the lock arg to wait_on_page_bit_common() to implement it.
> No interruptible or killable variant needed yet, but they might follow:
> I have a vague notion that reporting -EINTR should take precedence over
> return from wait_on_page_bit_common() without knowing the page state,
> so arrange it accordingly - but that may be nothing but pedantic.
>
> shrink_page_list()'s __ClearPageLocked(): that was a surprise! this
> survived a lot of testing before that showed up. It does raise the
> question: should is_page_cache_freeable() and __remove_mapping() now
> treat a PG_waiters page as if an extra reference were held? Perhaps,
> but I don't think it matters much, since shrink_page_list() already
> had to win its trylock_page(), so waiters are not very common there: I
> noticed no difference when trying the bigger change, and it's surely not
> needed while put_and_wait_on_page_locked() is only for page migration.
>
> Signed-off-by: Hugh Dickins <hughd@google.com>
> ---
...
> @@ -1100,6 +1111,17 @@ static inline int wait_on_page_bit_common(wait_queue_head_t *q,
> ret = -EINTR;
> break;
> }
> +
> + if (lock < 0) {
> + /*
> + * We can no longer safely access page->flags:
Hmm...
> + * even if CONFIG_MEMORY_HOTREMOVE is not enabled,
> + * there is a risk of waiting forever on a page reused
> + * for something that keeps it locked indefinitely.
> + * But best check for -EINTR above before breaking.
> + */
> + break;
> + }
> }
>
> finish_wait(q, wait);
... the code continues by:
if (thrashing) {
if (!PageSwapBacked(page))
So maybe we should not set 'thrashing' true when lock < 0?
Thanks!
Vlastimil
next prev parent reply other threads:[~2018-11-20 13:38 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-14 7:09 Memory hotplug softlock issue Baoquan He
2018-11-14 7:16 ` Baoquan He
2018-11-14 8:18 ` David Hildenbrand
2018-11-14 9:00 ` Baoquan He
2018-11-14 9:25 ` David Hildenbrand
2018-11-14 9:41 ` Michal Hocko
2018-11-14 9:48 ` David Hildenbrand
2018-11-14 10:04 ` Michal Hocko
2018-11-14 9:01 ` Michal Hocko
2018-11-14 9:22 ` David Hildenbrand
2018-11-14 9:37 ` Michal Hocko
2018-11-14 9:39 ` David Hildenbrand
2018-11-14 14:52 ` Baoquan He
2018-11-14 15:00 ` Michal Hocko
2018-11-15 5:10 ` Baoquan He
2018-11-15 7:30 ` Michal Hocko
2018-11-15 7:53 ` Baoquan He
2018-11-15 8:30 ` Michal Hocko
2018-11-15 9:42 ` David Hildenbrand
2018-11-15 9:52 ` Baoquan He
2018-11-15 9:53 ` David Hildenbrand
2018-11-15 13:12 ` Baoquan He
2018-11-15 13:19 ` Michal Hocko
2018-11-15 13:23 ` Baoquan He
2018-11-15 14:25 ` Michal Hocko
2018-11-15 13:38 ` Baoquan He
2018-11-15 14:32 ` Michal Hocko
2018-11-15 14:34 ` Baoquan He
2018-11-16 1:24 ` Baoquan He
2018-11-16 9:14 ` Michal Hocko
2018-11-17 4:22 ` Baoquan He
[not found] ` <20181119105202.GE18471@MiWiFi-R3L-srv>
2018-11-19 12:40 ` Michal Hocko
2018-11-19 12:51 ` Michal Hocko
2018-11-19 14:10 ` Michal Hocko
2018-11-19 16:36 ` Vlastimil Babka
2018-11-19 16:46 ` Michal Hocko
2018-11-19 16:46 ` Vlastimil Babka
2018-11-19 16:48 ` Vlastimil Babka
2018-11-19 17:01 ` Michal Hocko
2018-11-19 17:33 ` Michal Hocko
2018-11-19 20:34 ` Hugh Dickins
2018-11-19 20:59 ` Michal Hocko
2018-11-20 1:56 ` Baoquan He
2018-11-20 5:44 ` Hugh Dickins
2018-11-20 13:38 ` Vlastimil Babka [this message]
2018-11-20 13:58 ` Baoquan He
2018-11-20 14:05 ` Michal Hocko
2018-11-20 14:12 ` Baoquan He
2018-11-21 1:21 ` Hugh Dickins
2018-11-21 1:08 ` Hugh Dickins
2018-11-21 3:20 ` Hugh Dickins
2018-11-21 17:31 ` Michal Hocko
2018-11-22 1:53 ` Hugh Dickins
2018-11-14 10:00 ` Michal Hocko
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