On Sat, Dec 05 2015, Verma, Vishal L wrote:
> On Fri, 2015-12-04 at 15:30 -0800, James Bottomley wrote:
> [...]
>> > +ssize_t badblocks_store(struct badblocks *bb, const char *page,
>> > size_t len,
>> > + int unack)
>> [...]
>> > +int badblocks_init(struct badblocks *bb, int enable)
>> > +{
>> > + bb->count = 0;
>> > + if (enable)
>> > + bb->shift = 0;
>> > + else
>> > + bb->shift = -1;
>> > + bb->page = kmalloc(PAGE_SIZE, GFP_KERNEL);
>>
>> Why not __get_free_page(GFP_KERNEL)? The problem with kmalloc of an
>> exactly known page sized quantity is that the slab tracker for this
>> requires two contiguous pages for each page because of the overhead.
>
> Cool, I didn't know about __get_free_page - I can fix this up too.
>
I was reminded of this just recently I thought I should clear up the
misunderstanding.
kmalloc(PAGE_SIZE) does *not* incur significant overhead and certainly
does not require two contiguous free pages.
If you "grep kmalloc-4096 /proc/slabinfo" you will note that both
objperslab and pagesperslab are 1. So one page is used to store each
4096 byte allocation.
To quote the email from Linus which reminded me about this
> If you
> want to allocate a page, and get a pointer, just use "kmalloc()".
> Boom, done!
https://lkml.org/lkml/2015/12/21/605
There probably is a small CPU overhead from using kmalloc, but no memory
overhead.
NeilBrown
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