Re: [RFC PATCH 1/3] mm, reclaim: make should_continue_reclaim perform dryrun detection

[Mike Kravetz <mike.kravetz@oracle.com>]

On 7/31/19 4:08 AM, Vlastimil Babka wrote:
> 
> I agree this is an improvement overall, but perhaps the patch does too
> many things at once. The reshuffle is one thing and makes sense. The
> change of the last return condition could perhaps be separate. Also
> AFAICS the ultimate result is that when nr_reclaimed == 0, the function
> will now always return false. Which makes the initial test for
> __GFP_RETRY_MAYFAIL and the comments there misleading. There will no
> longer be a full LRU scan guaranteed - as long as the scanned LRU chunk
> yields no reclaimed page, we abort.

Can someone help me understand why nr_scanned == 0 guarantees a full
LRU scan?  FWICS, nr_scanned used in this context is only incremented
in shrink_page_list and potentially shrink_zones.  In the stall case I
am looking at, there are MANY cases in which nr_scanned is only a few
pages and none of those are reclaimed.

Can we not get nr_scanned == 0 on an arbitrary chunk of the LRU?

I must be missing something, because I do not see how nr_scanned == 0
guarantees a full scan.
-- 
Mike Kravetz