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

[Peter Xu]

On Tue, Sep 22, 2020 at 11:02:03AM -0700, John Hubbard wrote:
> On 9/22/20 8:17 AM, Peter Xu wrote:
> > 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..
> > 
> 
> That's just a bit of harmless redundancy, intended to help clarify where this
> is going. But if the redundancy isn't actually helping, you could simply
> truncate it to the first half of the sentence, like this:
> 
> "Future work is planned, to provide a more sophisticated solution, likely
> involving a counter."

Will do.  Thanks.

-- 
Peter Xu