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Date:   Sat, 16 Mar 2019 09:23:54 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Oscar Salvador <osalvador@suse.de>
Cc:     akpm@linux-foundation.org, anshuman.khandual@arm.com,
        william.kucharski@oracle.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Jan Kara <jack@suse.cz>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH] mm: Fix __dump_page when mapping->host is not set
Message-ID: <20190316082354.GF15672@dhcp22.suse.cz>
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 <20190315143304.pkuvj4qwtlzgm7iq@d104.suse.de>
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On Fri 15-03-19 15:33:07, Oscar Salvador wrote:
> On Fri, Mar 15, 2019 at 01:47:33PM +0100, Michal Hocko wrote:
> > diff --git a/mm/debug.c b/mm/debug.c
> > index 1611cf00a137..499c26d5ebe5 100644
> > --- a/mm/debug.c
> > +++ b/mm/debug.c
> > @@ -78,6 +78,9 @@ void __dump_page(struct page *page, const char *reason)
> >  	else if (PageKsm(page))
> >  		pr_warn("ksm ");
> >  	else if (mapping) {
> > +		if (PageSwapCache(page))
> > +			mapping = page_swap_info(page)->swap_file->f_mapping;
> > +
> >  		pr_warn("%ps ", mapping->a_ops);
> >  		if (mapping->host->i_dentry.first) {
> >  			struct dentry *dentry;
> 
> This looks like a much nicer fix, indeed.

If we go this way then we should swap the order and print the mapping
before we alter it.

> I gave it a spin and it works.

Thanks for testing!

> Since the mapping is set during the swapon, I would assume that this should
> always work for swap.
> Although I am not sure if once you start playing with e.g zswap the picture can
> change.
> 
> Let us wait for Hugh and Jan.

Yes, I really cannot tell this is really safe. Maybe we want to do the
check for host anyway. Just to be sure.
-- 
Michal Hocko
SUSE Labs