Linux-mm Archive on lore.kernel.org
 help / color / Atom feed
From: Balbir Singh <bsingharora@gmail.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Baoquan He <bhe@redhat.com>, Oscar Salvador <OSalvador@suse.com>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	Michal Hocko <mhocko@suse.com>
Subject: Re: [PATCH] mm, memory_hotplug: check zone_movable in has_unmovable_pages
Date: Wed, 7 Nov 2018 07:35:18 +1100
Message-ID: <20181106203518.GC9042@350D> (raw)
In-Reply-To: <20181106095524.14629-1-mhocko@kernel.org>

On Tue, Nov 06, 2018 at 10:55:24AM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> Page state checks are racy. Under a heavy memory workload (e.g. stress
> -m 200 -t 2h) it is quite easy to hit a race window when the page is
> allocated but its state is not fully populated yet. A debugging patch to
> dump the struct page state shows
> : [  476.575516] has_unmovable_pages: pfn:0x10dfec00, found:0x1, count:0x0
> : [  476.582103] page:ffffea0437fb0000 count:1 mapcount:1 mapping:ffff880e05239841 index:0x7f26e5000 compound_mapcount: 1
> : [  476.592645] flags: 0x5fffffc0090034(uptodate|lru|active|head|swapbacked)
> 
> Note that the state has been checked for both PageLRU and PageSwapBacked
> already. Closing this race completely would require some sort of retry
> logic. This can be tricky and error prone (think of potential endless
> or long taking loops).
> 
> Workaround this problem for movable zones at least. Such a zone should
> only contain movable pages. 15c30bc09085 ("mm, memory_hotplug: make
> has_unmovable_pages more robust") has told us that this is not strictly
> true though. Bootmem pages should be marked reserved though so we can
> move the original check after the PageReserved check. Pages from other
> zones are still prone to races but we even do not pretend that memory
> hotremove works for those so pre-mature failure doesn't hurt that much.
> 
> Reported-and-tested-by: Baoquan He <bhe@redhat.com>
> Acked-by: Baoquan He <bhe@redhat.com>
> Fixes: "mm, memory_hotplug: make has_unmovable_pages more robust")
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>
> Hi,
> this has been reported [1] and we have tried multiple things to address
> the issue. The only reliable way was to reintroduce the movable zone
> check into has_unmovable_pages. This time it should be safe also for
> the bug originally fixed by 15c30bc09085.
> 
> [1] http://lkml.kernel.org/r/20181101091055.GA15166@MiWiFi-R3L-srv
>  mm/page_alloc.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 863d46da6586..c6d900ee4982 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7788,6 +7788,14 @@ bool has_unmovable_pages(struct zone *zone, struct page *page, int count,
>  		if (PageReserved(page))
>  			goto unmovable;
>  
> +		/*
> +		 * If the zone is movable and we have ruled out all reserved
> +		 * pages then it should be reasonably safe to assume the rest
> +		 * is movable.
> +		 */
> +		if (zone_idx(zone) == ZONE_MOVABLE)
> +			continue;
> +
>  		/*


There is a WARN_ON() in case of failure at the end of the routine,
is that triggered when we hit the bug? If we're adding this patch,
the WARN_ON needs to go as well.

The check seems to be quite aggressive and in a loop that iterates
pages, but has nothing to do with the page, did you mean to make
the check

zone_idx(page_zone(page)) == ZONE_MOVABLE

it also skips all checks for pinned pages and other checks


Balbir Singh. 

  parent reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-06  9:55 Michal Hocko
2018-11-06 11:00 ` osalvador
2018-11-06 20:35 ` Balbir Singh [this message]
2018-11-07  7:35   ` Michal Hocko
2018-11-07  7:40     ` Michal Hocko
2018-11-07  7:55     ` osalvador
2018-11-07  8:14       ` Michal Hocko
2018-11-07 12:53     ` Balbir Singh
2018-11-07 13:06       ` Michal Hocko
2018-11-09 10:45         ` Balbir Singh
2018-11-15  3:13 ` Baoquan He
2018-11-15  3:18   ` Baoquan He

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20181106203518.GC9042@350D \
    --to=bsingharora@gmail.com \
    --cc=OSalvador@suse.com \
    --cc=akpm@linux-foundation.org \
    --cc=bhe@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@kernel.org \
    --cc=mhocko@suse.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-mm Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-mm/0 linux-mm/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-mm linux-mm/ https://lore.kernel.org/linux-mm \
		linux-mm@kvack.org
	public-inbox-index linux-mm

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kvack.linux-mm


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git