From mboxrd@z Thu Jan 1 00:00:00 1970 From: Will Deacon Subject: Re: [PATCH v3, RFC] iommu/io-pgtable-arm-v7s: Use page_frag to request DMA32 memory Date: Tue, 4 Dec 2018 16:41:42 +0000 Message-ID: <20181204164142.GA8520@arm.com> References: <20181204082300.95106-1-drinkcat@chromium.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20181204082300.95106-1-drinkcat-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: iommu-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: Nicolas Boichat Cc: Michal Hocko , Tomasz Figa , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Matthew Wilcox , linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Vlastimil Babka , Matthias Brugger , yingjoe.chen-NuS5LvNUpcJWk0Htik3J/w@public.gmane.org, Christoph Lameter , Robin Murphy , linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org List-Id: iommu@lists.linux-foundation.org On Tue, Dec 04, 2018 at 04:23:00PM +0800, Nicolas Boichat wrote: > IOMMUs using ARMv7 short-descriptor format require page tables > (level 1 and 2) to be allocated within the first 4GB of RAM, even > on 64-bit systems. > > For level 1/2 tables, ensure GFP_DMA32 is used if CONFIG_ZONE_DMA32 > is defined (e.g. on arm64 platforms). > > For level 2 tables (1 KB), we use page_frag to allocate these pages, > as we cannot directly use kmalloc (no slab cache for GFP_DMA32) or > kmem_cache (mm/ code treats GFP_DMA32 as an invalid flag). > > One downside is that we only free the allocated page if all the > 4 fragments (4 IOMMU L2 tables) are freed, but given that we > usually only allocate limited number of IOMMU L2 tables, this > should not have too much impact on memory usage: In the absolute > worst case (4096 L2 page tables, each on their own 4K page), > we would use 16 MB of memory for 4 MB of L2 tables. > > Also, print an error when the physical address does not fit in > 32-bit, to make debugging easier in the future. > > Fixes: ad67f5a6545f ("arm64: replace ZONE_DMA with ZONE_DMA32") > Signed-off-by: Nicolas Boichat > --- > > As an alternative to the series [1], which adds support for GFP_DMA32 > to kmem_cache in mm/. IMHO the solution in [1] is cleaner and more > efficient, as it allows freed fragments (L2 tables) to be reused, but > this approach does not require any core change. > > [1] https://patchwork.kernel.org/cover/10677529/, 3 patches > > drivers/iommu/io-pgtable-arm-v7s.c | 32 ++++++++++++++++-------------- > 1 file changed, 17 insertions(+), 15 deletions(-) > > diff --git a/drivers/iommu/io-pgtable-arm-v7s.c b/drivers/iommu/io-pgtable-arm-v7s.c > index 445c3bde04800c..0de6a51eb6755f 100644 > --- a/drivers/iommu/io-pgtable-arm-v7s.c > +++ b/drivers/iommu/io-pgtable-arm-v7s.c > @@ -161,6 +161,12 @@ > > #define ARM_V7S_TCR_PD1 BIT(5) > > +#ifdef CONFIG_ZONE_DMA32 > +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA32 > +#else > +#define ARM_V7S_TABLE_GFP_DMA GFP_DMA > +#endif We may as well include __GFP_ZERO in here too. Anyway, this looks alright to me: Acked-by: Will Deacon But it sounds like you're still on the fence about this patch, so I won't pick it up unless you ask explicitly. Will