Re: 4.2-rc2: hitting "file-max limit 8192 reached"

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 14 Jul 2015 08:54:11 -0700 Dave Hansen <dave.hansen@intel.com> wrote:

> My laptop has been behaving strangely with 4.2-rc2.  Once I log in to my
> X session, I start getting all kinds of strange errors from applications
> and see this in my dmesg:
> 
> 	VFS: file-max limit 8192 reached
> 
> Could this be from CONFIG_DEFERRED_STRUCT_PAGE_INIT=y?  files_init()
> seems top be sizing files_stat.max_files from memory sizes.

argh.

> vfs_caches_init() uses nr_free_pages() to figure out what the "current
> kernel size" is in early boot.  *But* since we have not freed most of
> our memory, nr_free_pages() is low and makes us calculate the reserve as
> if the kernel we huge.
> 
> Adding some printk's confirms this.  Broken kernel:
> 
> 	vfs_caches_init() mempages: 4026972
> 	vfs_caches_init() reserve: 4021629
> 	vfs_caches_init() mempages (after reserve minus): 5343
> 	files_init() n: 2137
> 	files_init() files_stat.max_files: 8192
> 
> Working kernel:
> 
> 	vfs_caches_init() mempages: 4026972
> 	vfs_caches_init() reserve: 375
> 	vfs_caches_init() mempages2: 4026597
> 	files_init() n: 1610638
> 	files_init() files_stat.max_files: 1610638
> 
> Do we have an alternative to call instead of nr_free_pages() in
> vfs_caches_init()?
> 
> I guess we could save off 'nr_initialized' in memmap_init_zone() and
> then use "nr_initialized - nr_free_pages()", but that seems a bit hackish.

There are a lot of things that might be affected this way.  Callers of
nr_free_buffer_pages(), nr_free_pagecache_pages(), etc.

If we'd fully used the memory hotplug infrastructure then everything
would work - all those knobs which are sized off free-memory would get
themselves resized as more memory comes on line.  But quite a few
things have been missed.

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