On Thu, Feb 15, 2018 at 10:59:47PM -0500, je...@suse.com wrote:
> From: Jeff Mahoney <je...@suse.com>
> The srcu_struct in btrfs_fs_infoa scales in size with NR_CPUS.  On
> kernels built with NR_CPUS=8192, this can result in kmalloc failures
> that prevent mounting.
> There is work in progress to try to resolve this for every user of
> srcu_struct but using kvzalloc will work around the failures until
> that is complete.

Interesting, the subvol_srcu is the worst contirbutor of the fs_info
size, on a config with NR_CPUS=512:

        struct srcu_struct         subvol_srcu;          /*  1064  3480 */
        /* size: 6496, cachelines: 102, members: 181 */

Using kvzalloc makes sense and is a minimal fix. In the longterm I'd
rather allocate subvol_rcu dynamically so the whole fs_info is not
vmalloced (in the rare case when kmalloc would not work). As this is
unpredictable and almost invisible, I'm worried about some random
effects (performance, virtual mappings), so it would be better to avoid
them if possible.

Reviewed-by: David Sterba <dste...@suse.com>
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