Re: [PATCH] crypto: testmgr - allocate buffers with __GFP_COMP

[Matthew Wilcox]

On Thu, Apr 11, 2019 at 01:32:32PM -0700, Kees Cook wrote:
> > @@ -156,7 +156,8 @@ static int __testmgr_alloc_buf(char *buf[XBUFSIZE], int order)
> >         int i;
> >
> >         for (i = 0; i < XBUFSIZE; i++) {
> > -               buf[i] = (char *)__get_free_pages(GFP_KERNEL, order);
> > +               buf[i] = (char *)__get_free_pages(GFP_KERNEL | __GFP_COMP,
> > +                                                 order);
> 
> Is there a reason __GFP_COMP isn't automatically included in all page
> allocations? (Or rather, it seems like the exception is when things
> should NOT be considered part of the same allocation, so something
> like __GFP_SINGLE should exist?.)

The question is not whether or not things should be considered part of the
same allocation.  The question is whether the allocation is of a compound
page or of N consecutive pages.  Now you're asking what the difference is,
and it's whether you need to be able to be able to call compound_head(),
compound_order(), PageTail() or use a compound_dtor.  If you don't, then
you can save some time at allocation & free by not specifying __GFP_COMP.

I'll agree this is not documented well, and maybe most multi-page
allocations do want __GFP_COMP and we should invert that bit, but
__GFP_SINGLE doesn't seem like the right antonym to __GFP_COMP to me.