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

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Date:   Sun, 11 Feb 2018 15:51:07 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Kai Heng Feng <kai.heng.feng@...onical.com>,
        Laura Abbott <labbott@...hat.com>, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, James.Bottomley@...senPartnership.com,
        davem@...hat.com
Subject: Re: Regression after commit 19809c2da28a ("mm, vmalloc: use
 __GFP_HIGHMEM implicitly")

On Sun, Feb 11, 2018 at 04:05:15AM -0800, Matthew Wilcox wrote:
> On Sun, Feb 11, 2018 at 03:28:08AM -0800, Matthew Wilcox wrote:
> > Now, longer-term, perhaps we should do the following:
> > 
> > #ifdef CONFIG_ZONE_DMA32
> > #define OPT_ZONE_DMA32	ZONE_DMA32
> > #elif defined(CONFIG_64BIT)
> > #define OPT_ZONE_DMA	OPT_ZONE_DMA
> > #else
> > #define OPT_ZONE_DMA32 ZONE_NORMAL
> > #endif
> 
> For consistent / coherent memory, we have an allocation function.
> But we don't have an allocation function for streaming memory, which is
> what these drivers want.  They also flush the DMA memory and then access
> the memory through a different virtual mapping, which I'm not sure is
> going to work well on virtually-indexed caches like SPARC and PA-RISC
> (maybe not MIPS either?)

Perhaps I (and a number of other people ...) have misunderstood the
semantics of GFP_DMA32.  Perhaps GFP_DMA32 is not "allocate memory below
4GB", perhaps it's "allocate memory which can be mapped below 4GB".
Machines with an IOMMU can use ZONE_NORMAL.  Machines with no IOMMU can
choose to allocate memory with a physical address below 4GB.

After all, it has 'DMA' right there in the name.  If someone's relying
on it to allocate physical memory below 4GB, they're arguably misusing it.