linux-kernel - Re: [fs] faf99b5635: will-it-scale.per_thread

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Message-ID: <16d100df-6d88-c7c5-9b1b-fea40ec40d96@fb.com>
Date:   Tue, 16 Aug 2022 11:35:44 +0200
From:   Stefan Roesch <shr@...com>
To:     Christian Brauner <brauner@...nel.org>,
        Christoph Hellwig <hch@....de>, Jan Kara <jack@...e.cz>
Cc:     Jens Axboe <axboe@...nel.dk>,
        "Darrick J. Wong" <djwong@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, lkp@...ts.01.org, lkp@...el.com,
        ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...ux.intel.com, fengwei.yin@...el.com,
        kernel test robot <oliver.sang@...el.com>
Subject: Re: [fs] faf99b5635: will-it-scale.per_thread_ops -9.0% regression


On 8/15/22 12:00 PM, Christian Brauner wrote:
> !-------------------------------------------------------------------|
>   This Message Is From an External Sender
> 
> |-------------------------------------------------------------------!
> 
> On Mon, Aug 15, 2022 at 12:32:25PM +0800, kernel test robot wrote:
>>
>>
>> Greeting,
>>
>> FYI, we noticed a -9.0% regression of will-it-scale.per_thread_ops due to commit:
>>
>>
>> commit: faf99b563558f74188b7ca34faae1c1da49a7261 ("fs: add __remove_file_privs() with flags parameter")
>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> This seems overall pretty odd tbh at least it's not immediately obvious
> how that specific commit would've caused this. But fwiw, I think there's
> one issue in this change which we originally overlooked which might
> explain this.
> 
> Before faf99b563558 ("fs: add __remove_file_privs() with flags
> parameter") inode_has_no_xattr() was called when
> dentry_needs_remove_privs() returned 0.
> 
> 	int error = 0
> 	[...]
> 	kill = dentry_needs_remove_privs(dentry);
> 	if (kill < 0)
> 		return kill;
> 	if (kill)
> 		error = __remove_privs(file_mnt_user_ns(file), dentry, kill);
> 	if (!error)
> 		inode_has_no_xattr(inode);
> 
> but now we do:
> 
> 	kill = dentry_needs_remove_privs(dentry);
> 	if (kill <= 0)
> 		return kill;
> 
> which means we don't call inode_has_no_xattr(). I don't think that we
> did this intentionally. inode_has_no_xattr() just sets S_NOSEC which
> means next time we call into __file_remove_privs() we can return earlier
> instead of hitting dentry_needs_remove_privs() again:
> 
> if (IS_NOSEC(inode) || !S_ISREG(inode->i_mode))
> 	return 0;
> 
> So I think that needs to be fixed?
> 

Christian,

thanks for looking into this. I'll prepare a fix to maintain the original behavior.

--Stefan

> Christian