linux-kernel - Re: [PATCH RFC v2 bpf-next 1/9] mm: Store build id in inode object

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Message-ID: <Y/8J7pkJ8g1uEQcq@krava>
Date:   Wed, 1 Mar 2023 09:16:46 +0100
From:   Jiri Olsa <olsajiri@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Hao Luo <haoluo@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Matthew Wilcox <willy@...radead.org>, bpf@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Namhyung Kim <namhyung@...il.com>
Subject: Re: [PATCH RFC v2 bpf-next 1/9] mm: Store build id in inode object

On Tue, Feb 28, 2023 at 11:13:10AM -0800, Andrew Morton wrote:
> On Tue, 28 Feb 2023 10:31:58 +0100 Jiri Olsa <jolsa@...nel.org> wrote:
> 
> > Storing build id in file's inode object for elf executable with build
> > id defined. The build id is stored when file is mmaped.
> > 
> > This is enabled with new config option CONFIG_INODE_BUILD_ID.
> > 
> > The build id is valid only when the file with given inode is mmap-ed.
> > 
> > We store either the build id itself or the error we hit during
> > the retrieval.
> > 
> > ...
> >
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -699,6 +700,12 @@ struct inode {
> >  	struct fsverity_info	*i_verity_info;
> >  #endif
> >  
> > +#ifdef CONFIG_INODE_BUILD_ID
> > +	/* Initialized and valid for executable elf files when mmap-ed. */
> > +	struct build_id		*i_build_id;
> > +	spinlock_t		i_build_id_lock;
> > +#endif
> > +
> 
> Remember we can have squillions of inodes in memory.  So that's one
> costly spinlock!
> 
> AFAICT this lock could be removed if mmap_region() were to use an
> atomic exchange on inode->i_build_id?

right, that should work I'll check 

> 
> If not, can we use an existing lock?  i_lock would be appropriate
> (don't forget to update its comment).

ok

> 
> Also, the code in mmap_region() runs build_id_free() inside the locked
> region, which seems unnecessary.
> 

ok, if the atomic exchange is doable, it'll take care of this

thanks,
jirka