On Wed, Dec 5, 2018 at 5:56 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > On Wed 05-12-18 13:48:27, Nicolas Boichat wrote: > > In some cases (e.g. IOMMU ARMv7s page allocator), we need to allocate > > data structures smaller than a page with GFP_DMA32 flag. > > > > This change makes it possible to create a custom cache in DMA32 zone > > using kmem_cache_create, then allocate memory using kmem_cache_alloc. > > > > We do not create a DMA32 kmalloc cache array, as there are currently > > no users of kmalloc(..., GFP_DMA32). The new test in check_slab_flags > > ensures that such calls still fail (as they do before this change). > > The changelog should be much more specific about decisions made here. > First of all it would be nice to mention the usecase. Ok, I'll copy most of the cover letter text here (i.e. the fact that IOMMU wants physical memory <4GB for L2 page tables, why it's better than genalloc/page_frag). > Secondly, why do we need a new sysfs file? Who is going to consume it? We have cache_dma, so it seems consistent to add cache_dma32. I wasn't aware of tools/vm/slabinfo.c, so I can add support for cache_dma32 in a follow-up patch. Any other user I should take care of? > Then why do we need SLAB_MERGE_SAME to cover GFP_DMA32 as well? SLAB_MERGE_SAME tells us which flags _need_ to be the same for the slabs to be merged. We don't want slab caches with GFP_DMA32 and ~GFP_DMA32 to be merged, so it should be in there. (https://elixir.bootlin.com/linux/v4.19.6/source/mm/slab_common.c#L342). > I > thought the whole point is to use dedicated slab cache. Who is this > going to merge with? Well, if there was another SLAB cache requiring 1KB GFP_DMA32 elements, then I don't see why we would not merge the caches. This is what happens with this IOMMU L2 tables cache pre-CONFIG_ZONE_DMA32 on arm64 (output on some 3.18 kernel below), and what would happen on arm32 since we still use GFP_DMA. /sys/kernel/slab # ls -l | grep dt-0001024 drwxr-xr-x. 2 root root 0 Dec 5 02:25 :dt-0001024 lrwxrwxrwx. 1 root root 0 Dec 5 02:25 dma-kmalloc-1024 -> :dt-0001024 lrwxrwxrwx. 1 root root 0 Dec 5 02:25 io-pgtable_armv7s_l2 -> :dt-0001024 Thanks! > -- > Michal Hocko > SUSE Labs
