linux-kernel - Re: [RFC PATCH 1/5] mm: print more information about mapping in __dump

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Date:   Sun, 25 Nov 2018 09:10:03 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, Oscar Salvador <OSalvador@...e.com>,
        Baoquan He <bhe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/5] mm: print more information about mapping in
 __dump_page

On Fri 23-11-18 16:04:04, Andrew Morton wrote:
> On Wed,  7 Nov 2018 11:18:26 +0100 Michal Hocko <mhocko@...nel.org> wrote:
> 
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > __dump_page prints the mapping pointer but that is quite unhelpful
> > for many reports because the pointer itself only helps to distinguish
> > anon/ksm mappings from other ones (because of lowest bits
> > set). Sometimes it would be much more helpful to know what kind of
> > mapping that is actually and if we know this is a file mapping then also
> > try to resolve the dentry name.
> > 
> > ...
> >
> > --- a/mm/debug.c
> > +++ b/mm/debug.c
> >
> > ...
> >
> > @@ -70,6 +71,18 @@ void __dump_page(struct page *page, const char *reason)
> >  	if (PageCompound(page))
> >  		pr_cont(" compound_mapcount: %d", compound_mapcount(page));
> >  	pr_cont("\n");
> > +	if (PageAnon(page))
> > +		pr_emerg("anon ");
> > +	else if (PageKsm(page))
> > +		pr_emerg("ksm ");
> > +	else if (mapping) {
> > +		pr_emerg("%ps ", mapping->a_ops);
> > +		if (mapping->host->i_dentry.first) {
> > +			struct dentry *dentry;
> > +			dentry = container_of(mapping->host->i_dentry.first, struct dentry, d_u.d_alias);
> > +			pr_emerg("name:\"%*s\" ", dentry->d_name.len, dentry->d_name.name);
> > +		}
> > +	}
> 
> There has to be a better way of printing the filename.  It is so often
> needed.
> 
> The (poorly named and gleefully undocumented)
> take_dentry_name_snapshot() looks promising.  However it's unclear that
> __dump_page() is always called from contexts where
> take_dentry_name_snapshot() and release_dentry_name_snapshot() can be
> safely called.  Probably it's OK, but how to guarantee it?

http://lkml.kernel.org/r/20181125080834.GB12455@dhcp22.suse.cz as
suggested by Tetsuo?
-- 
Michal Hocko
SUSE Labs