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

Andrew Morton
2023-02-28T19:13:17Z

On Tue, 28 Feb 2023 10:31:58 +0100 Jiri Olsa <jolsa@kernel.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?

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

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