Re: [PATCH 4/5] vrange: Set affected pages referenced when marking volatile

[KOSAKI Motohiro]

On Fri, Mar 21, 2014 at 2:17 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> One issue that some potential users were concerned about, was that
> they wanted to ensure that all the pages from one volatile range
> were purged before we purge pages from a different volatile range.
> This would prevent the case where they have 4 large objects, and
> the system purges one page from each object, casuing all of the
> objects to have to be re-created.
>
> The counter-point to this case, is when an application is using the
> SIGBUS semantics to continue to access pages after they have been
> marked volatile. In that case, the desire was that the most recently
> touched pages be purged last, and only the "cold" pages be purged
> from the specified range.
>
> Instead of adding option flags for the various usage model (at least
> initially), one way of getting a solutoin for both uses would be to
> have the act of marking pages as volatile in effect mark the pages
> as accessed. Since all of the pages in the range would be marked
> together, they would be of the same "age" and would (approximately)
> be purged together. Further, if any pages in the range were accessed
> after being marked volatile, they would be moved to the end of the
> lru and be purged later.

If you run after two hares, you will catch neither. I suspect this patch
doesn't make happy any user.
I suggest to aim former case (object level caching) and aim latter by
another patch-kit.
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