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Date:   Mon, 20 Apr 2020 21:12:55 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Jan Kara <jack@suse.cz>, Matthew Wilcox <willy@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Robert Stupp <snazy@gmx.de>
Subject: Re: [PATCH 2/3] mm: fix long time stall from mm_populate
Message-Id: <20200420211255.55bb6e1275cf90f9dc292238@linux-foundation.org>
In-Reply-To: <20200212221614.215302-2-minchan@kernel.org>
References: <20200212221614.215302-1-minchan@kernel.org>
        <20200212221614.215302-2-minchan@kernel.org>
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On Wed, 12 Feb 2020 14:16:13 -0800 Minchan Kim <minchan@kernel.org> wrote:

> Basically, fault handler releases mmap_sem before requesting readahead
> and then it is supposed to retry lookup the page from page cache with
> FAULT_FLAG_TRIED so that it avoids the live lock of infinite retry.
> 
> However, what happens if the fault handler find a page from page
> cache and the page has readahead marker but are waiting under
> writeback? Plus one more condition, it happens under mm_populate
> which repeats faulting unless it encounters error. So let's assemble
> conditions below.
> 
> __mm_populate
> for (...)
>   get_user_pages(faluty_address)
>     handle_mm_fault
>       filemap_fault
>         find a page form page(PG_uptodate|PG_readahead|PG_writeback)
> 	it will return VM_FAULT_RETRY
>   continue with faulty_address
> 
> IOW, it will repeat fault retry logic until the page will be written
> back in the long run. It makes big spike latency of several seconds.
> 
> This patch solves the issue by turning off fault retry logic in second
> trial.

I guess a resend would be helpful, to refresh memories.

Please mention in the changelog whether this "long time stall" has been
observed in practice.  If so, under what conditions, how often and how
long was it?  etcetera.