Re: [PATCH v3] inotify: Increase default inotify.max_user_watches limit to 1048576

[Waiman Long <longman@redhat.com>]

On 10/30/20 6:57 AM, Jan Kara wrote:
> On Thu 29-10-20 15:42:56, Waiman Long 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.
>>
>> 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 vfs inode have a size that
>> is a bit over 1 kbytes (1284 bytes with my x86-64 config).  That means
>> a system with 128GB or more memory will likely have the maximum value
>> of 1048576 for inotify.max_user_watches. This default should be big
>> enough for most use cases.
>>
>> [v3: increase inotify watch cost as suggested by Amir and Honza]
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
> Overall this looks fine. Some remaining nits below.
>
>> diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
>> index 186722ba3894..f8065eda3a02 100644
>> --- a/fs/notify/inotify/inotify_user.c
>> +++ b/fs/notify/inotify/inotify_user.c
>> @@ -37,6 +37,15 @@
>>   
>>   #include <asm/ioctls.h>
>>   
>> +/*
>> + * An inotify watch requires allocating an inotify_inode_mark structure as
>> + * well as pinning the watched inode. Doubling the size of a VFS inode
>> + * should be more than enough to cover the additional filesystem inode
>> + * size increase.
>> + */
>> +#define INOTIFY_WATCH_COST	(sizeof(struct inotify_inode_mark) + \
>> +				 2 * sizeof(struct inode))
>> +
>>   /* configurable via /proc/sys/fs/inotify/ */
>>   static int inotify_max_queued_events __read_mostly;
>>   
>> @@ -801,6 +810,18 @@ SYSCALL_DEFINE2(inotify_rm_watch, int, fd, __s32, wd)
>>    */
>>   static int __init inotify_user_setup(void)
>>   {
>> +	unsigned int watches_max;
>> +	struct sysinfo si;
>> +
>> +	si_meminfo(&si);
>> +	/*
>> +	 * Allow up to 1% of addressible memory to be allocated for inotify
> 			     ^^^^ addressable
>
>> +	 * watches (per user) limited to the range [8192, 1048576].
>> +	 */
>> +	watches_max = (((si.totalram - si.totalhigh) / 100) << PAGE_SHIFT) /
>> +			INOTIFY_WATCH_COST;
> 			^^^ So for machines with > 1TB of memory
> watches_max would overflow. So you probably need to use ulong for that.
>
>
>> +	watches_max = min(1048576U, max(watches_max, 8192U));
> 			^^^ use clamp() here?

Yes, it will be easier to read to use clamp() here. Will send out v4 
withat those changes.

Thanks,
Longman