Re: [PATCH v3 02/16] mm/compaction: support non-lru movable page migration

[Minchan Kim <minchan@kernel.org>]

On Mon, Apr 04, 2016 at 03:24:34PM +0200, Vlastimil Babka wrote:
> On 04/04/2016 07:12 AM, Minchan Kim wrote:
> >On Fri, Apr 01, 2016 at 11:29:14PM +0200, Vlastimil Babka wrote:
> >>Might have been better as a separate migration patch and then a
> >>compaction patch. It's prefixed mm/compaction, but most changed are
> >>in mm/migrate.c
> >
> >Indeed. The title is rather misleading but not sure it's a good idea
> >to separate compaction and migration part.
> 
> Guess it's better to see the new functions together with its user
> after all, OK.
> 
> >I will just resend to change the tile from "mm/compaction" to
> >"mm/migration".
> 
> OK!
> 
> >>Also I'm a bit uncomfortable how isolate_movable_page() blindly expects that
> >>page->mapping->a_ops->isolate_page exists for PageMovable() pages.
> >>What if it's a false positive on a PG_reclaim page? Can we rely on
> >>PG_reclaim always (and without races) implying PageLRU() so that we
> >>don't even attempt isolate_movable_page()?
> >
> >For now, we shouldn't have such a false positive because PageMovable
> >checks page->_mapcount == PAGE_MOVABLE_MAPCOUNT_VALUE as well as PG_movable
> >under PG_lock.
> >
> >But I read your question about user-mapped drvier pages so we cannot
> >use _mapcount anymore so I will find another thing. A option is this.
> >
> >static inline int PageMovable(struct page *page)
> >{
> >         int ret = 0;
> >         struct address_space *mapping;
> >         struct address_space_operations *a_op;
> >
> >         if (!test_bit(PG_movable, &(page->flags))
> >                 goto out;
> >
> >         mapping = page->mapping;
> >         if (!mapping)
> >                 goto out;
> >
> >         a_op = mapping->a_op;
> >         if (!aop)
> >                 goto out;
> >         if (a_op->isolate_page)
> >                 ret = 1;
> >out:
> >         return ret;
> >
> >}
> >
> >It works under PG_lock but with this, we need trylock_page to peek
> >whether it's movable non-lru or not for scanning pfn.
> 
> Hm I hoped that with READ_ONCE() we could do the peek safely without
> trylock_page, if we use it only as a heuristic. But I guess it would
> require at least RCU-level protection of the
> page->mapping->a_op->isolate_page chain.
> 
> >For avoiding that, we need another function to peek which just checks
> >PG_movable bit instead of all things.
> >
> >
> >/*
> >  * If @page_locked is false, we cannot guarantee page->mapping's stability
> >  * so just the function checks with PG_movable which could be false positive
> >  * so caller should check it again under PG_lock to check a_ops->isolate_page.
> >  */
> >static inline int PageMovable(struct page *page, bool page_locked)
> >{
> >         int ret = 0;
> >         struct address_space *mapping;
> >         struct address_space_operations *a_op;
> >
> >         if (!test_bit(PG_movable, &(page->flags))
> >                 goto out;
> >
> >         if (!page_locked) {
> >                 ret = 1;
> >                 goto out;
> >         }
> >
> >         mapping = page->mapping;
> >         if (!mapping)
> >                 goto out;
> >
> >         a_op = mapping->a_op;
> >         if (!aop)
> >                 goto out;
> >         if (a_op->isolate_page)
> >                 ret = 1;
> >out:
> >         return ret;
> >}
> 
> I wouldn't put everything into single function, but create something
> like __PageMovable() just for the unlocked peek. Unlike the
> zone->lru_lock, we don't keep page_lock() across iterations in
> isolate_migratepages_block(), as obviously each page has different
> lock.
> So the page_locked parameter would be always passed as constant, and
> at that point it's better to have separate functions.

Agree.

> 
> So I guess the question is how many false positives from overlap
> with PG_reclaim the scanner will hit if we give up on
> PAGE_MOVABLE_MAPCOUNT_VALUE, as that will increase number of page
> locks just to realize that it's not actual PageMovable() page...

I don't think it's too many because PG_reclaim bit is set to only
LRU pages at the moment and we can check PageMovable after !PageLRU
check.

Thanks.