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Date:   Sun, 14 Apr 2019 19:24:12 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Eric Biggers <ebiggers@kernel.org>,
        Rik van Riel <riel@surriel.com>,
        linux-crypto <linux-crypto@vger.kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Dmitry Vyukov <dvyukov@google.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Laura Abbott <labbott@redhat.com>, linux-mm@kvack.org
Subject: Re: [PATCH] crypto: testmgr - allocate buffers with __GFP_COMP
Message-ID: <20190415022412.GA29714@bombadil.infradead.org>
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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.