Re: [PATCH v2 0/3] iommu/io-pgtable-arm-v7s: Use DMA32 zone for page tables

[Nicolas Boichat]

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