Re: [PATCH 1/5] mm: Introduce mm_struct.has_pinned

From: John Hubbard
Date: Mon Sep 21 2020 - 20:01:23 EST

On 9/21/20 4:53 PM, John Hubbard wrote:
On 9/21/20 2:17 PM, Peter Xu wrote:
(Commit message collected from Jason Gunthorpe)

Reduce the chance of false positive from page_maybe_dma_pinned() by keeping

Not yet, it doesn't. :)  More:

track if the mm_struct has ever been used with pin_user_pages(). mm_structs
that have never been passed to pin_user_pages() cannot have a positive
page_maybe_dma_pinned() by definition. This allows cases that might drive up
the page ref_count to avoid any penalty from handling dma_pinned pages.

Due to complexities with unpining this trivial version is a permanent sticky
bit, future work will be needed to make this a counter.

How about this instead:

Subsequent patches intend to reduce the chance of false positives from
page_maybe_dma_pinned(), by also considering whether or not a page has
even been part of an mm struct that has ever had pin_user_pages*()

arggh, correction: please make that:

"...whether or not a page is part of an mm struct that...".

(Present tense.) Otherwise, people start wondering about the checkered past
of a page's past lives, and it badly distracts from the main point here. :)

John Hubbard