Re: [PATCH 3/5] cifsd: add file operations

[Matthew Wilcox <willy@infradead.org>]

On Mon, Mar 22, 2021 at 02:57:18PM +0100, Christoph Hellwig wrote:
> On Mon, Mar 22, 2021 at 06:03:21PM +0900, Sergey Senozhatsky wrote:
> > On (21/03/22 08:15), Matthew Wilcox wrote:
> > > 
> > > What's the scenario for which your allocator performs better than slub
> > > 
> > 
> > IIRC request and reply buffers can be up to 4M in size. So this stuff
> > just allocates a number of fat buffers and keeps them around so that
> > it doesn't have to vmalloc(4M) for every request and every response.
> 
> Do we have any data suggesting it is faster than vmalloc?

Oh, I have no trouble believing it's faster than vmalloc.  Here's
the fast(!) path that always has memory available, never does retries.
I'm calling out the things I perceive as expensive on the right hand side.
Also, I'm taking the 4MB size as the example.

vmalloc()
  __vmalloc_node()
    __vmalloc_node_range()
      __get_vm_area_node()
				[allocates vm_struct]
	alloc_vmap_area()
				[allocates vmap_area]
				[takes free_vmap_area_lock]
	  __alloc_vmap_area()
	    find_vmap_lowest_match
				[walks free_vmap_area_root]
				[takes vmap_area_lock]
      __vmalloc_area_node()
				... array_size is 8KiB, we call __vmalloc_node
	__vmalloc_node
				[everything we did above, all over again,
				 two more allocations, two more lock acquire]
	alloc_pages_node(), 1024 times
	vmap_pages_range_noflush()
	  vmap_range_noflush()
				[allocate at least two pages for PTEs]

There's definitely some low handling fruit here.  __vmalloc_area_node()
should probably call kvmalloc_node() instead of __vmalloc_node() for
table sizes > 4KiB.  But a lot of this is inherent to how vmalloc works,
and we need to put a cache in front of it.  Just not this one.