Date:   Wed, 15 Dec 2021 07:03:35 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Baoquan He <bhe@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, akpm@linux-foundation.org, hch@lst.de,
        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
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References: <20211213122712.23805-1-bhe@redhat.com>
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 <20211213134319.GA997240@odroid>
 <20211214053253.GB2216@MiWiFi-R3L-srv>
 <f5ff82eb-73b6-55b5-53d7-04ab73ce5035@suse.cz>
 <20211215044818.GB1097530@odroid>
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On Wed, Dec 15, 2021 at 04:48:26AM +0000, Hyeonggon Yoo wrote:
> 
> Hello Baoquan and Vlastimil.
> 
> I'm not sure allowing ZONE_DMA32 for kdump kernel is nice way to solve
> this problem. Devices that requires ZONE_DMA is rare but we still
> support them.
> 
> If we allow ZONE_DMA32 for ZONE_DMA in kdump kernels,
> the problem will be hard to find.
> 

Sorry, I sometimes forget validating my english writing :(

What I meant:

I'm not sure that allocating from ZONE_DMA32 instead of ZONE_DMA
for kdump kernel is nice way to solve this problem.

Devices that requires ZONE_DMA memory is rare but we still support them.

If we use ZONE_DMA32 memory instead of ZONE_DMA in kdump kernels,
It will be hard to the problem when we use devices that can use only
ZONE_DMA memory.

> What about one of those?:
> 
>     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
> 
> Thanks,
> Hyeonggon.
> 
> > >> 
> > >> Maybe the function get_capabilities() want to allocate memory
> > >> even if it's not from DMA zone, but other callers will not expect that.
> > > 
> > > 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.
> > > 
> > > 
> > >> 
> > >> >  			kmalloc_caches[KMALLOC_DMA][i] = create_kmalloc_cache(
> > >> >  				kmalloc_info[i].name[KMALLOC_DMA],
> > >> >  				kmalloc_info[i].size,
> > >> > -- 
> > >> > 2.17.2
> > >> > 
> > >> > 
> > >> 
> > > 
> >