From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1mzD9j-0012Xi-ON for kexec@lists.infradead.org; Mon, 20 Dec 2021 07:32:25 +0000 Date: Mon, 20 Dec 2021 15:32:10 +0800 From: Baoquan He Subject: Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone Message-ID: <20211220073210.GA31681@MiWiFi-R3L-srv> References: <20211213122712.23805-1-bhe@redhat.com> <20211213122712.23805-6-bhe@redhat.com> <20211213134319.GA997240@odroid> <20211214053253.GB2216@MiWiFi-R3L-srv> <20211215044818.GB1097530@odroid> <20211215070335.GA1165926@odroid> <20211215072710.GA3010@lst.de> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "kexec" Errors-To: kexec-bounces+dwmw2=infradead.org@lists.infradead.org To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Christoph Hellwig , Vlastimil Babka , 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 , David Rientjes , Joonsoo Kim On 12/17/21 at 11:38am, Hyeonggon Yoo wrote: > 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, > > Hmm I think step 1) will be needed if someone is allocating pages from > DMA zone not using kmalloc or DMA API. (for example directly allocating > from buddy allocator) is there such cases? I think Christoph meant to take off the warning. I will post a patch to mute the warning if it's requesting page from DMA zone which has no managed pages. > > > 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. > > > > That's good. this cleanup will also remove unnecessary limitations. I searched and investigated several callsites where kmalloc(GFP_DMA) is called. E.g drivers/scsi/sr.c: sr_probe(). The scsi sr driver doesn't check DMA supporting capibility at all, e.g the dma limit, to set the dma mask or coherent_dma_mask. If we want to convert the kmalloc(GFP_DMA) to dma_alloc* API, scsi sr drvier developer/expert's suggestion and help is necessary. Either someone who knows this well help to change it, or give suggestion how to change so that I can do it. > > > > > > > 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. > _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec