Re: [PATCH v1 00/15] Keep track of GUPed pages in fs and block

Boaz Harrosh
2019-04-16T19:28:47Z

On 16/04/19 22:12, Dan Williams wrote:
> On Tue, Apr 16, 2019 at 11:59 AM Kent Overstreet
> <kent.overstreet@gmail.com> wrote:
<>
> This all reminds of the failed attempt to teach the block layer to
> operate without pages:
>
> https://lore.kernel.org/lkml/20150316201640.33102.33761.stgit@dwillia2-desk3.amr.corp.intel.com/
>

Exactly why I want to make sure it is just a [pointer | flag] and not any kind of pfn
type. Let us please not go there again?

>>
>> Question though - why do we need a flag for whether a page is a GUP page or not?
>> Couldn't the needed information just be determined by what range the pfn is not
>> (i.e. whether or not it has a struct page associated with it)?
>
> That amounts to a pfn_valid() check which is a bit heavier than if we
> can store a flag in the bv_pfn entry directly.
>
> I'd say create a new PFN_* flag, and make bv_pfn a 'pfn_t' rather than
> an 'unsigned long'.
>

No, please please not. This is not a pfn and not a pfn_t. It is a page-ptr
and a flag that says where/how to put_page it. IE I did a GUP on this page
please do a PUP on this page instead of regular put_page. So no where do I mean
pfn or pfn_t in this code. Then why?

> That said, I'm still in favor of Jan's proposal to just make the
> bv_page semantics uniform. Otherwise we're complicating this core
> infrastructure for some yet to be implemented GPU memory management
> capabilities with yet to be determined value. Circle back when that
> value is clear, but in the meantime fix the GUP bug.
>

I agree there are simpler ways to solve the bugs at hand then
to system wide separate get_user_page from get_page and force all put_user
callers to remember what to do. Is there some Document explaining the
all design of where this is going?

Thanks
Boaz