linux-kernel - Re: [PATCH V3 3/7] mm: reclaim MADV

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Date:   Fri, 17 Feb 2017 11:15:32 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Shaohua Li <shli@...com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Kernel-team@...com, mhocko@...e.com,
        hughd@...gle.com, riel@...hat.com, mgorman@...hsingularity.net,
        akpm@...ux-foundation.org
Subject: Re: [PATCH V3 3/7] mm: reclaim MADV_FREE pages

On Fri, Feb 17, 2017 at 02:41:08PM +0900, Minchan Kim wrote:
> Hi Johannes,
> 
> On Thu, Feb 16, 2017 at 01:40:18PM -0500, Johannes Weiner wrote:
> > On Tue, Feb 14, 2017 at 11:36:09AM -0800, Shaohua Li wrote:
> > > @@ -911,7 +911,7 @@ static void page_check_dirty_writeback(struct page *page,
> > >  	 * Anonymous pages are not handled by flushers and must be written
> > >  	 * from reclaim context. Do not stall reclaim based on them
> > >  	 */
> > > -	if (!page_is_file_cache(page)) {
> > > +	if (!page_is_file_cache(page) || page_is_lazyfree(page)) {
> > 
> > Do we need this? MADV_FREE clears the dirty bit off the page; we could
> > just let them go through with the function without any special-casing.
> 
> I thought some driver potentially can do GUP with FOLL_TOUCH so that the
> lazyfree page can have PG_dirty with !PG_swapbacked. In this case,
> throttling logic of shrink_page_list can be confused?

Yep, agreed. We should filter these pages here.

> > > @@ -1142,7 +1144,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > >  		 * The page is mapped into the page tables of one or more
> > >  		 * processes. Try to unmap it here.
> > >  		 */
> > > -		if (page_mapped(page) && mapping) {
> > > +		if (page_mapped(page) && (mapping || lazyfree)) {
> > 
> > Do we actually need to filter for mapping || lazyfree? If we fail to
> > allocate swap, we don't reach here. If the page is a truncated file
> > page, ttu returns pretty much instantly with SWAP_AGAIN. We should be
> > able to just check for page_mapped() alone, no?
> 
> try_to_unmap_one assumes every anonymous pages reached will have swp_entry
> so it should be changed to check PageSwapCache if we go to the way.

Yep, I think it should check page_mapping(). To me that would make the
most sense, see other email: "Don't unmap a ram page with valid data
when there is no secondary storage mapping to maintain integrity."