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Date:   Wed, 12 Feb 2020 10:28:51 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     Jan Kara <jack@suse.cz>, Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: fix long time stall from mm_populate
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On Wed, Feb 12, 2020 at 09:40:15AM -0800, Minchan Kim wrote:
> How about this?
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 1bf83c8fcaa7..d07d602476df 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -363,8 +363,28 @@ PAGEFLAG(MappedToDisk, mappedtodisk, PF_NO_TAIL)
>  /* PG_readahead is only used for reads; PG_reclaim is only for writes */
>  PAGEFLAG(Reclaim, reclaim, PF_NO_TAIL)
>  	TESTCLEARFLAG(Reclaim, reclaim, PF_NO_TAIL)
> -PAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
> -	TESTCLEARFLAG(Readahead, reclaim, PF_NO_COMPOUND)
> +
> +SETPAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
> +CLEARPAGEFLAG(Readahead, reclaim, PF_NO_COMPOUND)
> +
> +/*
> + * Since PG_readahead is shared with PG_reclaim of the page flags,
> + * PageReadahead should double check whether it's readahead marker
> + * or PG_reclaim. It could be done by PageWriteback check because
> + * PG_reclaim is always with PG_writeback.
> + */
> +static inline int PageReadahead(struct page *page)
> +{
> +	VM_BUG_ON_PGFLAGS(PageCompound(page), page);
> +	return test_bit(PG_reclaim, &(page)->flags) && !PageWriteback(page);

Why not ...

	return page->flags & (1UL << PG_reclaim | 1UL << PG_writeback) ==
		(1UL << PG_reclaim);

> +static inline int TestClearPageReadahead(struct page *page)
> +{
> +	VM_BUG_ON_PGFLAGS(PageCompound(page), page);
> +
> +	return test_and_clear_bit(PG_reclaim, &page->flags) && !PageWriteback(page);

That's definitely wrong.  It'll clear PageReclaim and then pretend it did
nothing wrong.

	return !PageWriteback(page) ||
		test_and_clear_bit(PG_reclaim, &page->flags);