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Subject: Re: [PATCH 5.10] fget: clarify and improve __fget_files()
 implementation
To:     Greg KH <gregkh@linuxfoundation.org>
CC:     <linux-kernel@vger.kernel.org>, <stable@vger.kernel.org>,
        <oliver.sang@intel.com>, <beibei.si@intel.com>, <jannh@google.com>,
        <mszeredi@redhat.com>, <torvalds@linux-foundation.org>,
        <yukuai3@huawei.com>, Baokun Li <libaokun1@huawei.com>
References: <20220215065107.3045023-1-libaokun1@huawei.com>
 <Yg6aLQCR3zxn5XoW@kroah.com>
From:   "libaokun (A)" <libaokun1@huawei.com>
Message-ID: <f39e907b-203b-ded4-e0e5-0d89cd7eb3f6@huawei.com>
Date:   Fri, 18 Feb 2022 09:27:38 +0800
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在 2022/2/18 2:55, Greg KH 写道:
> On Tue, Feb 15, 2022 at 02:51:07PM +0800, Baokun Li wrote:
>> From: Linus Torvalds <torvalds@linux-foundation.org>
>>
>> commit e386dfc56f837da66d00a078e5314bc8382fab83 upstream.
>>
>> Commit 054aa8d439b9 ("fget: check that the fd still exists after getting
>> a ref to it") fixed a race with getting a reference to a file just as it
>> was being closed.  It was a fairly minimal patch, and I didn't think
>> re-checking the file pointer lookup would be a measurable overhead,
>> since it was all right there and cached.
>>
>> But I was wrong, as pointed out by the kernel test robot.
>>
>> The 'poll2' case of the will-it-scale.per_thread_ops benchmark regressed
>> quite noticeably.  Admittedly it seems to be a very artificial test:
>> doing "poll()" system calls on regular files in a very tight loop in
>> multiple threads.
>>
>> That means that basically all the time is spent just looking up file
>> descriptors without ever doing anything useful with them (not that doing
>> 'poll()' on a regular file is useful to begin with).  And as a result it
>> shows the extra "re-check fd" cost as a sore thumb.
>>
>> Happily, the regression is fixable by just writing the code to loook up
>> the fd to be better and clearer.  There's still a cost to verify the
>> file pointer, but now it's basically in the noise even for that
>> benchmark that does nothing else - and the code is more understandable
>> and has better comments too.
>>
>> [ Side note: this patch is also a classic case of one that looks very
>>    messy with the default greedy Myers diff - it's much more legible with
>>    either the patience of histogram diff algorithm ]
>>
>> Link: https://lore.kernel.org/lkml/20211210053743.GA36420@xsang-OptiPlex-9020/
>> Link: https://lore.kernel.org/lkml/20211213083154.GA20853@linux.intel.com/
>> Reported-by: kernel test robot <oliver.sang@intel.com>
>> Tested-by: Carel Si <beibei.si@intel.com>
>> Cc: Jann Horn <jannh@google.com>
>> Cc: Miklos Szeredi <mszeredi@redhat.com>
>> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
>> ---
>>   fs/file.c | 72 ++++++++++++++++++++++++++++++++++++++++++-------------
>>   1 file changed, 56 insertions(+), 16 deletions(-)
> Now queued up, thanks.
Thanks!
>
> Any chance you can do this for 5.4 and older kernels too?
It's my pleasure. I'll sync this patch to 5.4, 4.19, 4.14, 4.9, 4.4.
>
> greg k-h
> .

-- 
With Best Regards,
Baokun Li

.