linux-kernel - Re: Regression after commit 19809c2da28a ("mm, vmalloc: use __GFP

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Date:   Thu, 8 Feb 2018 15:20:04 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Kai Heng Feng <kai.heng.feng@...onical.com>
Cc:     Michal Hocko <mhocko@...e.com>, Laura Abbott <labbott@...hat.com>,
        linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Regression after commit 19809c2da28a ("mm, vmalloc: use
 __GFP_HIGHMEM implicitly")

On Thu, Feb 08, 2018 at 05:06:49AM -0800, Matthew Wilcox wrote:
> On Thu, Feb 08, 2018 at 02:29:57PM +0800, Kai Heng Feng wrote:
> > A user with i386 instead of AMD64 machine reports [1] that commit 19809c2da28a ("mm, vmalloc: use __GFP_HIGHMEM implicitly”) causes a regression.
> > BUG_ON(PageHighMem(pg)) in drivers/media/common/saa7146/saa7146_core.c always gets triggered after that commit.
> 
> Well, the BUG_ON is wrong.  You can absolutely have pages which are both
> HighMem and under the 4GB boundary.  Only the first 896MB (iirc) are LowMem,
> and the next 3GB of pages are available to vmalloc_32().

... nevertheless, 19809c2da28a does in fact break vmalloc_32 on 32-bit.  Look:

#if defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA32)
#define GFP_VMALLOC32 GFP_DMA32 | GFP_KERNEL
#elif defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA)
#define GFP_VMALLOC32 GFP_DMA | GFP_KERNEL
#else
#define GFP_VMALLOC32 GFP_KERNEL
#endif

So we pass in GFP_KERNEL to __vmalloc_node, which calls __vmalloc_node_range
which calls __vmalloc_area_node, which ORs in __GFP_HIGHMEM.

So ... we could enable ZONE_DMA32 on 32-bit architectures.  I don't know
what side-effects that might have; it's clearly only been tested on 64-bit
architectures so far.

It might be best to just revert 19809c2da28a and the follow-on 704b862f9efd.