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Date:   Mon, 10 Feb 2020 19:54:12 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Minchan Kim <minchan@kernel.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: <20200211035412.GR8731@bombadil.infradead.org>
References: <20200211001958.170261-1-minchan@kernel.org>
 <20200211011021.GP8731@bombadil.infradead.org>
 <20200211035004.GA242563@google.com>
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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.

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