Re: [PATCH] btrfs: use kvzalloc to allocate btrfs_fs_info

[Nikolay Borisov <nborisov@suse.com>]

On 19.02.2018 17:33, David Sterba wrote:
> On Thu, Feb 15, 2018 at 10:59:47PM -0500, jeffm@suse.com wrote:
>> From: Jeff Mahoney <jeffm@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.

The interesting bit is the "there is WIP trying to address this". Are
there any patches that have been sent to lkml?


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