Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 10 Feb 2020 20:25:36 -0800
From:   Minchan Kim <minchan@kernel.org>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Jan Kara <jack@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: fix long time stall from mm_populate
Message-ID: <20200211042536.GB242563@google.com>
References: <20200211001958.170261-1-minchan@kernel.org>
 <20200211011021.GP8731@bombadil.infradead.org>
 <20200211035004.GA242563@google.com>
 <20200211035412.GR8731@bombadil.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20200211035412.GR8731@bombadil.infradead.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Feb 10, 2020 at 07:54:12PM -0800, Matthew Wilcox wrote:
> On Mon, Feb 10, 2020 at 07:50:04PM -0800, Minchan Kim wrote:
> > On Mon, Feb 10, 2020 at 05:10:21PM -0800, Matthew Wilcox wrote:
> > > On Mon, Feb 10, 2020 at 04:19:58PM -0800, Minchan Kim wrote:
> > > >       filemap_fault
> > > >         find a page form page(PG_uptodate|PG_readahead|PG_writeback)
> > > 
> > > Uh ... That shouldn't be possible.
> > 
> > Please see shrink_page_list. Vmscan uses PG_reclaim to accelerate
> > page reclaim when the writeback is done so the page will have both
> > flags at the same time and the PG reclaim could be regarded as
> > PG_readahead in fault conext.
> 
> What part of fault context can make that mistake?  The snippet I quoted
> below is from page_cache_async_readahead() where it will clearly not
> make that mistake.  There's a lot of code here; please don't presume I
> know all the areas you're talking about.

Sorry about being not clear. I am saying  filemap_fault ->
do_async_mmap_readahead

Let's assume the page is hit in page cache and vmf->flags is !FAULT_FLAG
TRIED so it calls do_async_mmap_readahead. Since the page has PG_reclaim
and PG_writeback by shrink_page_list, it goes to 

do_async_mmap_readahead
  if (PageReadahead(page))
    fpin = maybe_unlock_mmap_for_io();
    page_cache_async_readahead
      if (PageWriteback(page))
        return;
      ClearPageReadahead(page); <- doesn't reach here until the writeback is clear
      
So, mm_populate will repeat the loop until the writeback is done.
It's my just theory but didn't comfirm it by the testing.
If I miss something clear, let me know it.

Thanks!

> 
> > > 
> > >         /*
> > >          * Same bit is used for PG_readahead and PG_reclaim.
> > >          */
> > >         if (PageWriteback(page))
> > >                 return;
> > > 
> > >         ClearPageReadahead(page);