Re: [PATCH 1/2] kernfs: Add option to enable user xattrs

From: Tejun Heo <tj@kernel.org>
To: Daniel Xu <dxu@dxuuu.xyz>
Cc: cgroups@vger.kernel.org, lizefan@huawei.com, hannes@cmpxchg.org,
	linux-kernel@vger.kernel.org, gregkh@linuxfoundation.org,
	kernel-team@fb.com
Subject: Re: [PATCH 1/2] kernfs: Add option to enable user xattrs
Date: Wed, 4 Mar 2020 11:39:22 -0500	[thread overview]
Message-ID: <20200304163922.GG189690@mtj.thefacebook.com> (raw)
In-Reply-To: <C11GC2SN5D18.2S00I3KONE9ZE@dlxu-fedora-R90QNFJV>

Hello,

On Tue, Mar 03, 2020 at 11:37:28AM -0800, Daniel Xu wrote:
> It looks like in fs/xattr.c:setxattr, there is already:
> 
>     ...
>     if (size) {
>         if (size > XATTR_SIZE_MAX)
>             return -E2BIG;
>     ...
> 
> where XATTR_SIZE_MAX is defined as 64k. Do you want it even smaller?

Oh, I missed that. Order 5 allocations can still be on the big side
tho. Ideally, something like the following?

* Total number of bytes limit as the primary limit so that we can say
  that for a given cgroup user xattrs don't consume more than X bytes.
  We can pick an arbitrary number which is large enough for most use
  cases but not too big. e.g. 128k or whatever.

* Total number of xattrs limit. Again, some arbitrary not too low, not
  too high limit.

* Switch to kvmalloc() for attr allocation so that we don't have to
  worry about high order allocations.

Thanks.

-- 
tejun