Re: [PATCH v2] inotify: Increase default inotify.max_user_watches limit to 1048576

[Waiman Long <longman@redhat.com>]

On 10/29/20 2:46 PM, Amir Goldstein wrote:
> On Thu, Oct 29, 2020 at 8:05 PM Waiman Long <longman@redhat.com> wrote:
>> On 10/29/20 1:27 PM, Amir Goldstein wrote:
>>> On Thu, Oct 29, 2020 at 5:46 PM Waiman Long <longman@redhat.com> wrote:
>>>> The default value of inotify.max_user_watches sysctl parameter was set
>>>> to 8192 since the introduction of the inotify feature in 2005 by
>>>> commit 0eeca28300df ("[PATCH] inotify"). Today this value is just too
>>>> small for many modern usage. As a result, users have to explicitly set
>>>> it to a larger value to make it work.
>>>>
>>>> After some searching around the web, these are the
>>>> inotify.max_user_watches values used by some projects:
>>>>    - vscode:  524288
>>>>    - dropbox support: 100000
>>>>    - users on stackexchange: 12228
>>>>    - lsyncd user: 2000000
>>>>    - code42 support: 1048576
>>>>    - monodevelop: 16384
>>>>    - tectonic: 524288
>>>>    - openshift origin: 65536
>>>>
>>>> Each watch point adds an inotify_inode_mark structure to an inode to
>>>> be watched. It also pins the watched inode as well as an inotify fdinfo
>>>> procfs file.
>>>>
>>>> Modeled after the epoll.max_user_watches behavior to adjust the default
>>>> value according to the amount of addressable memory available, make
>>>> inotify.max_user_watches behave in a similar way to make it use no more
>>>> than 1% of addressable memory within the range [8192, 1048576].
>>>>
>>>> For 64-bit archs, inotify_inode_mark plus 2 inode have a size close
>>>> to 2 kbytes. That means a system with 196GB or more memory should have
>>>> the maximum value of 1048576 for inotify.max_user_watches. This default
>>>> should be big enough for most use cases.
>>>>
>>>> With my x86-64 config, the size of xfs_inode, proc_inode and
>>>> inotify_inode_mark is 1680 bytes. The estimated INOTIFY_WATCH_COST is
>>>> 1760 bytes.
>>>>
>>>> [v2: increase inotify watch cost as suggested by Amir and Honza]
>>>>
>>>> Signed-off-by: Waiman Long <longman@redhat.com>
>>>> ---
>>>>    fs/notify/inotify/inotify_user.c | 24 +++++++++++++++++++++++-
>>>>    1 file changed, 23 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
>>>> index 186722ba3894..37d9f09c226f 100644
>>>> --- a/fs/notify/inotify/inotify_user.c
>>>> +++ b/fs/notify/inotify/inotify_user.c
>>>> @@ -37,6 +37,16 @@
>>>>
>>>>    #include <asm/ioctls.h>
>>>>
>>>> +/*
>>>> + * An inotify watch requires allocating an inotify_inode_mark structure as
>>>> + * well as pinning the watched inode and adding inotify fdinfo procfs file.
>>> Maybe you misunderstood me.
>>> There is no procfs file per watch.
>>> There is a procfs file per inotify_init() fd.
>>> The fdinfo of that procfile lists all the watches of that inotify instance.
>> Thanks for the clarification. Yes, I probably had misunderstood you
>> because of the 2 * sizeof(inode) figure you provided.
>>>> + * The increase in size of a filesystem inode versus a VFS inode varies
>>>> + * depending on the filesystem. An extra 512 bytes is added as rough
>>>> + * estimate of the additional filesystem inode cost.
>>>> + */
>>>> +#define INOTIFY_WATCH_COST     (sizeof(struct inotify_inode_mark) + \
>>>> +                                2 * sizeof(struct inode) + 512)
>>>> +
>>> I would consider going with double the sizeof inode as rough approximation for
>>> filesystem inode size.
>>>
>>> It is a bit less arbitrary than 512 and it has some rationale behind it -
>>> Some kernel config options will grow struct inode (debug, smp)
>>> The same config options may also grow the filesystem part of the inode.
>>>
>>> And this approximation can be pretty accurate at times.
>>> For example, on Ubuntu 18.04 kernel 5.4.0:
>>> inode_cache        608
>>> nfs_inode_cache      1088
>>> btrfs_inode            1168
>>> xfs_inode              1024
>>> ext4_inode_cache   1096
>> Just to clarify, is your original 2 * sizeof(struct inode) figure
>> include the filesystem inode overhead or there is an additional inode
>> somewhere that I needs to go to 4 * sizeof(struct inode)?
> No additional inode.
>
> #define INOTIFY_WATCH_COST     (sizeof(struct inotify_inode_mark) + \
>                                                        2 * sizeof(struct inode))
>
> Not sure if the inotify_inode_mark part matters, but it doesn't hurt.
> Do note that Jan had a different proposal for fs inode size estimation (1K).
> I have no objection to this estimation if Jan insists.
>
> Thanks,
> Amir.
>
Thanks for the confirmation. 2*sizeof(struct inode) is more than 1k. 
Besides with debugging turned on, the size will increase more. So that 
figure is good enough.

Cheers,
Longman