Re: [PATCH v4] mm: Add PM_HUGE_THP_MAPPING to /proc/pid/pagemap

Peter Xu
2021-11-16T01:59:22Z

On Mon, Nov 15, 2021 at 02:50:26PM -0800, Mina Almasry wrote:
> PM_THP_MAPPED sounds good to me.
>
> TBH I think I still prefer this approach because it's a very simple 2
> line patch which addresses the concrete use case I have well. I'm not
> too familiar with the smaps code to be honest but I think adding a
> range-based smaps API will be a sizeable patch to add a syscall,
> handle a stable interface, and handle cases where the memory range
> doesn't match a VMA boundary. I'm not sure the performance benefit
> would justify this patch and I'm not sure the extra info from smaps
> would be widely useful. However if you insist and folks believe this
> is the better approach I can prototype a range-based smaps and test
> its performance to see if it works for us as well, just let me know
> what kind of API you're envisioning.

Yeah indeed I haven't yet thought enough on such a new interface, it's just
that I think it'll be something that solves a broader range of requests
including the thp-aware issue, so I raised it up.

That shouldn't require a lot code change either afaiu, as smap_gather_stats()
already takes a "start" and I think what's missing is another end where we just
pass in 0 when we want the default vma->vm_end as the end of range.

I don't have a solid clue on other use case to ask for that more generic
interface, so please feel free to move on with it. If you'll need a repost to
address the comment from Andrew on removing the debugging lines, please also
consider using the shorter PM_THP_MAPPED then it looks good to me too.

Thanks!

--
Peter Xu