On Sun, Apr 14, 2019 at 07:24:12PM -0700, Matthew Wilcox wrote:
> 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.
Thanks for clarifying Matthew.
Eric, this means that we should not use __GFP_COMP here just to
silent what is clearly a broken warning.
Cheers,
--
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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