linux-kernel - Re: [PATCH v2] fs: Do not check if there is a fsnotify watcher on pseudo inodes

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Message-ID: <4f6c8dab-4b54-d523-8636-2b01c03d14d3@gmail.com>
Date:   Mon, 29 Jun 2020 08:17:02 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] fs: Do not check if there is a fsnotify watcher on
 pseudo inodes



On 6/16/20 12:47 AM, Jan Kara wrote:
> On Mon 15-06-20 19:26:38, Amir Goldstein wrote:
>>> This patch changes alloc_file_pseudo() to always opt out of fsnotify by
>>> setting FMODE_NONOTIFY flag so that no check is made for fsnotify watchers
>>> on pseudo files. This should be safe as the underlying helper for the
>>> dentry is d_alloc_pseudo which explicitly states that no lookups are ever
>>> performed meaning that fanotify should have nothing useful to attach to.
>>>
>>> The test motivating this was "perf bench sched messaging --pipe". On
>>> a single-socket machine using threads the difference of the patch was
>>> as follows.
>>>
>>>                               5.7.0                  5.7.0
>>>                             vanilla        nofsnotify-v1r1
>>> Amean     1       1.3837 (   0.00%)      1.3547 (   2.10%)
>>> Amean     3       3.7360 (   0.00%)      3.6543 (   2.19%)
>>> Amean     5       5.8130 (   0.00%)      5.7233 *   1.54%*
>>> Amean     7       8.1490 (   0.00%)      7.9730 *   2.16%*
>>> Amean     12     14.6843 (   0.00%)     14.1820 (   3.42%)
>>> Amean     18     21.8840 (   0.00%)     21.7460 (   0.63%)
>>> Amean     24     28.8697 (   0.00%)     29.1680 (  -1.03%)
>>> Amean     30     36.0787 (   0.00%)     35.2640 *   2.26%*
>>> Amean     32     38.0527 (   0.00%)     38.1223 (  -0.18%)
>>>
>>> The difference is small but in some cases it's outside the noise so
>>> while marginal, there is still some small benefit to ignoring fsnotify
>>> for files allocated via alloc_file_pseudo in some cases.
>>>
>>> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
>>
>> Reviewed-by: Amir Goldstein <amir73il@...il.com>
> 
> Thanks for the patch Mel and for review Amir! I've added the patch to my
> tree with small amendments to the changelog.
> 
> 								Honza
> 

Note this patch broke some user programs (one instance being packetdrill)

Typical legacy packetdrill script has :

// Create a socket and set it to non-blocking.
    0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
   +0 fcntl(3, F_GETFL) = 0x2 (flags O_RDWR)
   +0 fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK) = 0


But after this change, fcntl(3, F_GETFL) returns 0x4000002 

FMODE_NONOTIFY was not meant to be visible to user space. (otherwise
there would be a O_NONOTIFY) ?