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

[Vlastimil Babka]

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
> list.
>
> 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.