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

[Minchan Kim]

On Mon, Mar 14, 2016 at 09:48:33AM +0100, Vlastimil Babka wrote:
> 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.

Good note. Although we cannot remove lru pvec draining completely,
at least, this patch removes a case which should drain pvec for
returning freed page to buddy.

Thanks for the notice.

> 
> >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.

I thought about it and we might do it via page_freeze_refs but
what I want at this moment is to separte two semantic put and putback.
;-)

> 
> >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.

Thanks for the review!