Re: [PATCH v1 01/19] mm: use put_page to free page instead of putback_lru_page

From: Vlastimil Babka
Date: Mon Mar 14 2016 - 04:48:55 EST

On 03/11/2016 08:30 AM, Minchan Kim wrote:
Procedure of page migration is as follows:

First of all, it should isolate a page from LRU and try to
migrate the page. If it is successful, it releases the page
for freeing. Otherwise, it should put the page back to LRU

For LRU pages, we have used putback_lru_page for both freeing
and putback to LRU list. It's okay because put_page is aware of
LRU list so if it releases last refcount of the page, it removes
the page from LRU list. However, It makes unnecessary operations
(e.g., lru_cache_add, pagevec and flags operations.

Yeah, and compaction (perhaps also other migration users) has to drain the lru pvec... Getting rid of this stuff is worth even by itself.

It would be
not significant but no worth to do) and harder to support new
non-lru page migration because put_page isn't aware of non-lru
page's data structure.

To solve the problem, we can add new hook in put_page with
PageMovable flags check but it can increase overhead in
hot path and needs new locking scheme to stabilize the flag check
with put_page.

So, this patch cleans it up to divide two semantic(ie, put and putback).
If migration is successful, use put_page instead of putback_lru_page and
use putback_lru_page only on failure. That makes code more readable
and doesn't add overhead in put_page.

I had an idea of checking for count==1 in putback_lru_page() which would take the put_page() shortcut from there. But maybe it can't be done nicely without races.

Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

Note in -next/after 4.6-rc1 this will need some rebasing though.