From: Hyeonggon Yoo <42.hyeyoo@gmail.com>
To: Christoph Hellwig <hch@lst.de>
Cc: Vlastimil Babka <vbabka@suse.cz>, Baoquan He <bhe@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
akpm@linux-foundation.org, cl@linux.com,
John.p.donnelly@oracle.com, kexec@lists.infradead.org,
stable@vger.kernel.org, Pekka Enberg <penberg@kernel.org>,
David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone
Date: Fri, 7 Jan 2022 11:56:38 +0000 [thread overview]
Message-ID: <20220107115638.GB2769814@odroid> (raw)
In-Reply-To: <20211215072710.GA3010@lst.de>
On Wed, Dec 15, 2021 at 08:27:10AM +0100, Christoph Hellwig wrote:
> On Wed, Dec 15, 2021 at 07:03:35AM +0000, Hyeonggon Yoo wrote:
> > I'm not sure that allocating from ZONE_DMA32 instead of ZONE_DMA
> > for kdump kernel is nice way to solve this problem.
>
> What is the problem with zones in kdump kernels?
>
> > Devices that requires ZONE_DMA memory is rare but we still support them.
>
> Indeed.
>
> > > 1) Do not call warn_alloc in page allocator if will always fail
> > > to allocate ZONE_DMA pages.
> > >
> > >
> > > 2) let's check all callers of kmalloc with GFP_DMA
> > > if they really need GFP_DMA flag and replace those by DMA API or
> > > just remove GFP_DMA from kmalloc()
> > >
> > > 3) Drop support for allocating DMA memory from slab allocator
> > > (as Christoph Hellwig said) and convert them to use DMA32
> >
> > (as Christoph Hellwig said) and convert them to use *DMA API*
> >
> > > and see what happens
>
> This is the right thing to do, but it will take a while. In fact
> I dont think we really need the warning in step 1, a simple grep
> already allows to go over them. I just looked at the uses of GFP_DMA
> in drivers/scsi for example, and all but one look bogus.
>
> > > > > Yeah, I have the same guess too for get_capabilities(), not sure about other
> > > > > callers. Or, as ChristophL and ChristophH said(Sorry, not sure if this is
> > > > > the right way to call people when the first name is the same. Correct me if
> > > > > it's wrong), any buffer requested from kmalloc can be used by device driver.
> > > > > Means device enforces getting memory inside addressing limit for those
> > > > > DMA transferring buffer which is usually large, Megabytes level with
> > > > > vmalloc() or alloc_pages(), but doesn't care about this kind of small
> > > > > piece buffer memory allocated with kmalloc()? Just a guess, please tell
> > > > > a counter example if anyone happens to know, it could be easy.
>
> The way this works is that the dma_map* calls will bounce buffer memory
> that does to fall into the addressing limitations. This is a performance
> overhead, but allows drivers to address all memory in a system. If the
> driver controls memory allocation it should use one of the dma_alloc_*
> APIs that allocate addressable memory from the start. The allocator
> will dip into ZONE_DMA and ZONE_DMA32 when needed.
Hello Christoph, Baoquan and I started this cleanup.
But we're a bit confused. I want to ask you something.
- Did you mean dma_map_* can handle arbitrary buffer, (and dma_map_* will
bounce buffer when necessary) Can we assume it on every architectures
and buses?
Reading at the DMA API documentation and code (dma_map_page_attrs(),
dma_direct_map_page()), I'm not sure about that.
In the documentation: (dma_map_single)
Further, the DMA address of the memory must be within the
dma_mask of the device (the dma_mask is a bit mask of the
addressable region for the device, i.e., if the DMA address of
the memory ANDed with the dma_mask is still equal to the DMA
address, then the device can perform DMA to the memory). To
ensure that the memory allocated by kmalloc is within the dma_mask,
the driver may specify various platform-dependent flags to restrict
the DMA address range of the allocation (e.g., on x86, GFP_DMA
guarantees to be within the first 16MB of available DMA addresses,
as required by ISA devices).
- In what function does the DMA API do bounce buffering?
Thanks a lot,
Hyeonggon
next prev parent reply other threads:[~2022-01-07 11:57 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-13 12:27 [PATCH v3 0/5] Avoid requesting page from DMA zone when no managed pages Baoquan He
2021-12-13 12:27 ` [PATCH v3 1/5] docs: kernel-parameters: Update to reflect the current default size of atomic pool Baoquan He
2021-12-13 14:20 ` john.p.donnelly
2021-12-13 12:27 ` [PATCH v3 2/5] dma-pool: allow user to disable " Baoquan He
2021-12-13 14:21 ` john.p.donnelly
2021-12-13 12:27 ` [PATCH v3 3/5] mm_zone: add function to check if managed dma zone exists Baoquan He
2021-12-13 14:22 ` john.p.donnelly
2021-12-16 10:52 ` David Hildenbrand
2021-12-13 12:27 ` [PATCH v3 4/5] dma/pool: create dma atomic pool only if dma zone has managed pages Baoquan He
2021-12-13 14:23 ` john.p.donnelly
2021-12-13 12:27 ` [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone Baoquan He
2021-12-13 13:43 ` Hyeonggon Yoo
2021-12-14 5:32 ` Baoquan He
2021-12-14 10:09 ` Vlastimil Babka
2021-12-14 10:28 ` Christoph Lameter
2021-12-15 4:48 ` Hyeonggon Yoo
2021-12-15 7:03 ` Hyeonggon Yoo
2021-12-15 7:27 ` Christoph Hellwig
2021-12-15 10:34 ` Vlastimil Babka
2021-12-15 11:51 ` David Laight
2021-12-15 13:41 ` Baoquan He
2021-12-17 11:38 ` Hyeonggon Yoo
2021-12-20 7:32 ` Baoquan He
2022-01-07 11:56 ` Hyeonggon Yoo [this message]
2021-12-15 14:42 ` Baoquan He
2021-12-15 10:08 ` Baoquan He
2021-12-17 11:38 ` Hyeonggon Yoo
2021-12-21 8:56 ` Christoph Hellwig
2021-12-22 12:37 ` Hyeonggon Yoo
2021-12-23 8:52 ` Christoph Hellwig
2021-12-13 14:24 ` john.p.donnelly
2021-12-14 16:31 ` Christoph Hellwig
2021-12-14 17:07 ` john.p.donnelly
2021-12-15 7:27 ` Christoph Hellwig
2021-12-13 21:05 ` [PATCH v3 0/5] Avoid requesting page from DMA zone when no managed pages Andrew Morton
2021-12-14 0:35 ` Baoquan He
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