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! _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
