linux-kernel - Re: [UBI UBIFS] replace vmalloc with kmalloc

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Message-ID: <20090807094404.GA10603@flint.arm.linux.org.uk>
Date:	Fri, 7 Aug 2009 10:44:04 +0100
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Adrian Hunter <adrian.hunter@...ia.com>
Cc:	JiSheng Zhang <jszhang3@...il.com>,
	"linux-mtd@...ts.infradead.org" <linux-mtd@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dwmw2@...radead.org" <dwmw2@...radead.org>,
	"Bityutskiy Artem (Nokia-D/Helsinki)" <Artem.Bityutskiy@...ia.com>
Subject: Re: [UBI UBIFS] replace vmalloc with kmalloc

On Fri, Aug 07, 2009 at 12:20:03PM +0300, Adrian Hunter wrote:
> vmalloc allows large (> 128KiB) buffers, but kmalloc doesn't.
> So we presently have no choice but to use vmalloc.
>
> I do not know what hardware you have or exactly what driver you
> are using, but we have UBIFS on OneNAND using DMA.
> See drivers/mtd/onenand/omap2.c

Hmm.  Looking at that code, it's unsafe:

        if (buf >= high_memory) {
                struct page *p1;

                if (((size_t)buf & PAGE_MASK) !=
                    ((size_t)(buf + count - 1) & PAGE_MASK))
                        goto out_copy;
                p1 = vmalloc_to_page(buf);
                if (!p1)
                        goto out_copy;
                buf = page_address(p1) + ((size_t)buf & ~PAGE_MASK);
        }
...
        dma_dst = dma_map_single(&c->pdev->dev, buf, count, DMA_FROM_DEVICE);

If you consider a VIVT cache, and with vmalloc'd pages you're passing
a *different* virtual address to the DMA functions, it's not going to
touch the cachelines associated with the vmalloc mapping.

The above *may* work for VIPT caches provided both the vmalloc and
kernel direct mappings are of the same colour.  If not, this really
isn't going to be reliable for the same reason.

Basically, what's going in there is *totally* unsafe.  I hope you
don't place important data on this NAND device.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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