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

On Tue, Dec 4, 2018 at 4:58 PM John Hubbard [off-list ref] wrote:

On 12/4/18 3:03 PM, Dan Williams wrote:

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On Tue, Dec 4, 2018 at 1:56 PM John Hubbard [off-list ref] wrote:

[..]

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Ok, sorry, I mis-remembered. So, you're effectively trying to capture
the end of the page pin event separate from the final 'put' of the
page? Makes sense.

Yes, that's it exactly.

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I was not able to actually find any place where a single additional page
bit would help our situation, which is why this still uses LRU fields for
both the two bits required (the RFC [1] still applies), and the dma_pinned_count.

Except the LRU fields are already in use for ZONE_DEVICE pages... how
does this proposal interact with those?

Very badly: page->pgmap and page->hmm_data both get corrupted. Is there an entire
use case I'm missing: calling get_user_pages() on ZONE_DEVICE pages? Said another
way: is it reasonable to disallow calling get_user_pages() on ZONE_DEVICE pages?

If we have to support get_user_pages() on ZONE_DEVICE pages, then the whole
LRU field approach is unusable.

Unfortunately, the entire motivation for ZONE_DEVICE was to support
get_user_pages() for persistent memory.