Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[John Hubbard <jhubbard@nvidia.com>]

On 12/7/18 9:18 PM, Matthew Wilcox wrote:
> On Fri, Dec 07, 2018 at 04:52:42PM -0800, John Hubbard wrote:
>> I see. OK, HMM has done an efficient job of mopping up unused fields, and now we are
>> completely out of space. At this point, after thinking about it carefully, it seems clear
>> that it's time for a single, new field:
> 
> Sorry for not replying earlier; I'm travelling and have had trouble
> keeping on top of my mail.

Hi Matthew,

> 
> Adding this field will grow struct page by 4-8 bytes, so it will no
> longer be 64 bytes.  This isn't an acceptable answer.

I had to ask, though, just in case the historical rules might no longer
be ask pressing. But OK.

> 
> We have a few options for bits.  One is that we have (iirc) two
> bits available in page->flags on 32-bit.  That'll force a few more
> configurations into using _last_cpupid and/or page_ext.  I'm not a huge
> fan of this approach.
> 
> The second is to use page->lru.next bit 1.  This requires some care
> because m68k allows misaligned pointers.  If the list_head that it's
> joined to is misaligned, we'll be in trouble.  This can get tricky because
> some pages are attached to list_heads which are on the stack ... and I
> don't think gcc guarantees __aligned attributes work for stack variables.
> 
> The third is to use page->lru.prev bit 0.  We'd want to switch pgmap
> and hmm_data around to make this work, and we'd want to record this
> in mm_types.h so nobody tries to use a field which aliases with
> page->lru.prev and has bit 0 set on a page which can be mapped to
> userspace (which I currently believe to be true).
> 
> The fourth is to use a bit in page->flags for 64-bit and a bit in
> page_ext->flags for 32-bit.  Or we could get rid of page_ext and grow
> struct page with a ->flags2 on 32-bit.
> 
> Fifth, it isn't clear to me how many bits might be left in ->_last_cpupid
> at this point, and perhaps there's scope for using a bit in there.
> 

Thanks for taking the time to collect and explain all of this, I'm stashing
it away as I'm sure it will come up again.

The latest approach to the gup/dma problem here might, or might not, actually
need a single page bit. I'll know in a day or two.

-- 
thanks,
John Hubbard
NVIDIA