On Fri, Nov 23, 2018 at 11:04 AM Nicolas Boichat <drinkcat@xxxxxxxxxxxx> wrote: > > On Thu, Nov 22, 2018 at 4:23 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > > > > On Wed, Nov 21, 2018 at 10:26:26PM +0000, Robin Murphy wrote: > > > TBH, if this DMA32 stuff is going to be contentious we could possibly just > > > rip out the offending kmem_cache - it seemed like good practice for the > > > use-case, but provided kzalloc(SZ_1K, gfp | GFP_DMA32) can be relied upon to > > > give the same 1KB alignment and chance of succeeding as the equivalent > > > kmem_cache_alloc(), then we could quite easily make do with that instead. > > > > Neither is the slab support for kmalloc, not do kmalloc allocations > > have useful alignment apparently (at least if you use slub debug). > > > > But I do agree with the sentiment of not wanting to spread GFP_DMA32 > > futher into the slab allocator. > > > > I think you want a simple genalloc allocator for this rather special > > use case. > > So I had a look at genalloc, we'd need to add pre-allocated memory > using gen_pool_add [1]. There can be up to 4096 L2 page tables, so we > may need to pre-allocate 4MB of memory (1KB per L2 page table). We > could add chunks on demand, but then it'd be difficult to free them up > (genalloc does not have a "gen_pool_remove" call). So basically if the > full 4MB end up being requested, we'd be stuck with that until the > iommu domain is freed (on the arm64 Mediatek platforms I looked at, > there is only one iommu domain, and it never gets freed). I tried out genalloc with pre-allocated 4MB, and that seems to work fine. Allocating in chunks would require genalloc changes as gen_pool_add calls kmalloc with just GFP_KERNEL [2], and we are in atomic context in __arm_v7s_alloc_table... [2] https://elixir.bootlin.com/linux/latest/source/lib/genalloc.c#L190 > page_frag would at least have a chance to reclaim those pages (if I > understand Christoph's statement correctly) > > Robin: Do you have some ideas of the lifetime/usage of L2 tables? If > they are usually few of them, or if they don't get reclaimed easily, > some on demand genalloc allocation would be ok (or even 4MB allocation > on init, if we're willing to take that hit). If they get allocated and > freed together, maybe page_frag is a better option? > > Thanks, > > [1] https://www.kernel.org/doc/html/v4.19/core-api/genalloc.html
