Re: [patch 6/12] hold atomic kmaps across generic_file_read

[Jamie Lokier <lk@tantalophile.demon.co.uk>]

Linus Torvalds wrote:
> Imagine doing a
> 
> 	fstat(fd..)
> 	buf = aligned_malloc(st->st_size)
> 	read(fd, buf, st->st_size);
> 
> and having it magically populate the VM directly with the whole file
> mapping, with _one_ failed page fault. And the above is actually a fairly
> common thing. See how many people have tried to optimize using mmap vs
> read, and what they _all_ really wanted was this "populate the pages in
> one go" thing. 

This will only provide the performance benefic when `aligned_malloc'
return "fresh" memory, i.e. memory that has never been written to.

Assuming most programs use plain old `malloc', which could be taught to
align nicely, then the optimisation might occur when a program starts
up, but later on it's more likely to return memory which has been
written to and previously freed.  So the performance becomes unpredictable.

But it's a nice way to optimise if you are _deliberately_ optimising a
user space program.  First call mmap() to get some fresh pages, then
call read() to fill them.  Slower on kernels without the optimisation,
fast on kernels with it. :-)

-- Jamie