linux-kernel - Re: [PATCH v3 2/2] mm: rid swapoff of quadratic complexity

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Date:   Tue, 1 Jan 2019 20:16:28 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Vineeth Pillai <vpillai@...italocean.com>
cc:     Hugh Dickins <hughd@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kelley Nielsen <kelleynnn@...il.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v3 2/2] mm: rid swapoff of quadratic complexity

On Tue, 1 Jan 2019, Vineeth Pillai wrote:

> Thanks a lot for the fixes and detailed explanation Hugh! I shall fold all
> the changes from you and Huang in the next iteration.
> 
> Thanks for all the suggestions and comments as well. I am looking into all
> those and will include all the changes in the next version. Will discuss
> over mail in case of any clarifications.

One more fix on top of what I sent yesterday: once I delved into
the retries, I found that the major cause of exceeding MAX_RETRIES
was the way the retry code neatly avoided retrying the last part of
its work.  With this fix in, I have not yet seen retries go above 1:
no doubt it could, but at present I have no actual evidence that
the MAX_RETRIES-or-livelock issue needs to be dealt with urgently.
Fix sent for completeness, but it reinforces the point that the
structure of try_to_unuse() should be reworked, and oldi gone.

Hugh

---

 mm/swapfile.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- mmotm/mm/swapfile.c	2018-12-31 12:30:55.822407154 -0800
+++ linux/mm/swapfile.c	2019-01-01 19:50:34.377277830 -0800
@@ -2107,8 +2107,8 @@ int try_to_unuse(unsigned int type, bool
 	struct swap_info_struct *si = swap_info[type];
 	struct page *page;
 	swp_entry_t entry;
-	unsigned int i = 0;
-	unsigned int oldi = 0;
+	unsigned int i;
+	unsigned int oldi;
 	int retries = 0;
 
 	if (!frontswap)
@@ -2154,6 +2154,7 @@ retry:
 		goto out;
 	}
 
+	i = oldi = 0;
 	while ((i = find_next_to_unuse(si, i, frontswap)) != 0) {
 		/*
 		 * Under global memory pressure, swap entries