On Mon, Sep 21, 2020 at 04:53:38PM -0700, 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*()
applied to any of its pages.
In order to allow that, provide a boolean value (even though it's not
implemented exactly as a boolean type) within the mm struct, that is
simply set once and never cleared. This will suffice for an early, rough
implementation that fixes a few problems.
Future work is planned, to provide a more sophisticated solution, likely
involving a counter, and *not* involving something that is set and never
cleared.
This looks good, thanks. Though I think Jason's version is good too (as long
as we remove the confusing sentence, that's the one starting with "mm_structs
that have never been passed... "). Before I drop Jason's version, I think I'd
better figure out what's the major thing we missed so that maybe we can add
another paragraph. E.g., "future work will be needed to make this a counter"
already means "involving a counter, and *not* involving something that is set
and never cleared" to me... Because otherwise it won't be called a counter..