linux-kernel - Re: [PATCH] crypto: testmgr - allocate buffers with __GFP

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Date:   Mon, 15 Apr 2019 10:46:15 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        linux-crypto <linux-crypto@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH] crypto: testmgr - allocate buffers with __GFP_COMP

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@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt