LinuxPPC-Dev Archive on lore.kernel.org
 help / color / Atom feed
From: David Laight <David.Laight@ACULAB.COM>
To: 'Geert Uytterhoeven' <geert@linux-m68k.org>,
	Matthew Wilcox <willy@infradead.org>
Cc: Arnd Bergmann <arnd@kernel.org>,
	Grygorii Strashko <grygorii.strashko@ti.com>,
	netdev <netdev@vger.kernel.org>,
	Ilias Apalodimas <ilias.apalodimas@linaro.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
	Michal Hocko <mhocko@kernel.org>, Linux MM <linux-mm@kvack.org>,
	Mel Gorman <mgorman@suse.de>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	"mcroce@linux.microsoft.com" <mcroce@linux.microsoft.com>,
	arcml <linux-snps-arc@lists.infradead.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Christoph Hellwig <hch@lst.de>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
Date: Tue, 20 Apr 2021 08:39:51 +0000
Message-ID: <bb8805a33371468892a7271dbe321e1f@AcuMS.aculab.com> (raw)
In-Reply-To: <CAMuHMdXm1Zg=Wm-=tn5jUJwqVGUvCi5yDaW0PXWC2DEDYGcy5A@mail.gmail.com>

From: Geert Uytterhoeven
> Sent: 20 April 2021 08:40
> 
> Hi Willy,
> 
> On Sat, Apr 17, 2021 at 4:49 AM Matthew Wilcox <willy@infradead.org> wrote:
> > Replacement patch to fix compiler warning.
> >
> > 32-bit architectures which expect 8-byte alignment for 8-byte integers
> > and need 64-bit DMA addresses (arc, arm, mips, ppc) had their struct
> > page inadvertently expanded in 2019.  When the dma_addr_t was added,
> > it forced the alignment of the union to 8 bytes, which inserted a 4 byte
> > gap between 'flags' and the union.
> >
> > Fix this by storing the dma_addr_t in one or two adjacent unsigned longs.
> > This restores the alignment to that of an unsigned long, and also fixes a
> > potential problem where (on a big endian platform), the bit used to denote
> > PageTail could inadvertently get set, and a racing get_user_pages_fast()
> > could dereference a bogus compound_head().
> >
> > Fixes: c25fff7171be ("mm: add dma_addr_t to struct page")
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> 
> Thanks for your patch!
> 
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -97,10 +97,10 @@ struct page {
> >                 };
> >                 struct {        /* page_pool used by netstack */
> >                         /**
> > -                        * @dma_addr: might require a 64-bit value even on
> > +                        * @dma_addr: might require a 64-bit value on
> >                          * 32-bit architectures.
> >                          */
> > -                       dma_addr_t dma_addr;
> > +                       unsigned long dma_addr[2];
> 
> So we get two 64-bit words on 64-bit platforms, while only one is
> needed?
> 
> Would
> 
>     unsigned long _dma_addr[sizeof(dma_addr_t) / sizeof(unsigned long)];
> 
> work?
> 
> Or will the compiler become too overzealous, and warn about the use
> of ...[1] below, even when unreachable?
> I wouldn't mind an #ifdef instead of an if () in the code below, though.

You could use [ARRAY_SIZE()-1] instead of [1].
Or, since IIRC it is the last member of that specific struct, define as:
		unsigned long dma_addr[];

...
> > -       return page->dma_addr;
> > +       dma_addr_t ret = page->dma_addr[0];
> > +       if (sizeof(dma_addr_t) > sizeof(unsigned long))
> > +               ret |= (dma_addr_t)page->dma_addr[1] << 16 << 16;
> 
> We don't seem to have a handy macro for a 32-bit left shift yet...
> 
> But you can also avoid the warning using
> 
>     ret |= (u64)page->dma_addr[1] << 32;

Or:
	ret |= page->dma_addr[1] + 0ull << 32;

Which relies in integer promotion rather than a cast.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  reply index

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-16 23:07 [PATCH 0/2] Change struct page layout for page_pool Matthew Wilcox (Oracle)
2021-04-16 23:07 ` [PATCH 1/2] mm: Fix struct page layout on 32-bit systems Matthew Wilcox (Oracle)
2021-04-17  1:08   ` kernel test robot
2021-04-17  2:45   ` Matthew Wilcox
2021-04-17 18:32     ` Ilias Apalodimas
2021-04-17 20:22       ` Matthew Wilcox
2021-04-18 11:21         ` Ilias Apalodimas
2021-04-17 21:18     ` David Laight
2021-04-17 22:45       ` Matthew Wilcox
2021-04-20  2:48     ` Vineet Gupta
2021-04-20  3:10       ` Matthew Wilcox
2021-04-20  7:07         ` Arnd Bergmann
2021-04-20 21:14           ` Vineet Gupta
2021-04-20 21:20             ` Arnd Bergmann
2021-04-21  5:50               ` hch
2021-04-21  8:43               ` David Laight
2021-04-21  8:58                 ` Arnd Bergmann
2021-04-20  7:39     ` Geert Uytterhoeven
2021-04-20  8:39       ` David Laight [this message]
2021-04-20 11:32       ` Matthew Wilcox
2021-04-17  7:34   ` Jesper Dangaard Brouer
2021-04-20  7:21   ` Rasmus Villemoes
2021-04-16 23:07 ` [PATCH 2/2] mm: Indicate pfmemalloc pages in compound_head Matthew Wilcox (Oracle)
2021-04-17 21:13   ` David Laight
2021-04-17 21:28     ` Matthew Wilcox
2021-04-20  4:03 ` [PATCH 0/2] Change struct page layout for page_pool Michael Ellerman

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=bb8805a33371468892a7271dbe321e1f@AcuMS.aculab.com \
    --to=david.laight@aculab.com \
    --cc=arnd@kernel.org \
    --cc=brouer@redhat.com \
    --cc=geert@linux-m68k.org \
    --cc=grygorii.strashko@ti.com \
    --cc=hch@lst.de \
    --cc=ilias.apalodimas@linaro.org \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mips@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-snps-arc@lists.infradead.org \
    --cc=linuxppc-dev@lists.ozlabs.org \
    --cc=mcroce@linux.microsoft.com \
    --cc=mgorman@suse.de \
    --cc=mhocko@kernel.org \
    --cc=netdev@vger.kernel.org \
    --cc=willy@infradead.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LinuxPPC-Dev Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linuxppc-dev/0 linuxppc-dev/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linuxppc-dev linuxppc-dev/ https://lore.kernel.org/linuxppc-dev \
		linuxppc-dev@lists.ozlabs.org linuxppc-dev@ozlabs.org
	public-inbox-index linuxppc-dev

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.ozlabs.lists.linuxppc-dev


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git