linux-kernel - Re: [RFC PATCH v2 10/16] mm,hwpoison: Rework soft offline for free pages

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Date:   Mon, 21 Oct 2019 14:58:49 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     n-horiguchi@...jp.nec.com, mike.kravetz@...cle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 10/16] mm,hwpoison: Rework soft offline for free
 pages

On Fri, Oct 18, 2019 at 02:06:15PM +0200, Michal Hocko wrote:
> On Thu 17-10-19 16:21:17, Oscar Salvador wrote:
> [...]
> > +bool take_page_off_buddy(struct page *page)
> > + {
> > +	struct zone *zone = page_zone(page);
> > +	unsigned long pfn = page_to_pfn(page);
> > +	unsigned long flags;
> > +	unsigned int order;
> > +	bool ret = false;
> > +
> > +	spin_lock_irqsave(&zone->lock, flags);
> 
> What prevents the page to be allocated in the meantime? Also what about
> free pages on the pcp lists? Also the page could be gone by the time you
> have reached here.

Nothing prevents the page to be allocated in the meantime.
We would just bail out and return -EBUSY to userspace.
Since we do not do __anything__ to the page until we are sure we took it off,
and it is completely isolated from the memory, there is no danger.

Since soft-offline is kinda "best effort" mode, it is something like:
"Sorry, could not poison the page, try again".

Now, thinking about this a bit more, I guess we could be more clever here
and call the routine that handles in-use pages if we see that the page
was allocated by the time we reach take_page_off_buddy.

About pcp pages, you are right.
I thought that we were already handling that case, and we do, but looking closer the
call to shake_page() (that among other things spills pcppages into buddy)
is performed at a later stage.
I think we need to adjust __get_any_page to recognize pcp pages as well.

I will do some work here.

Thanks for comments.

-- 
Oscar Salvador
SUSE L3