linux-kernel - Re: [PATCH 3/3] mm: make PageReadahead more strict

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Message-ID: <20200217093128.GB12032@quack2.suse.cz>
Date:   Mon, 17 Feb 2020 10:31:28 +0100
From:   Jan Kara <jack@...e.cz>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Josef Bacik <josef@...icpanda.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Robert Stupp <snazy@....de>
Subject: Re: [PATCH 3/3] mm: make PageReadahead more strict

On Wed 12-02-20 14:16:14, Minchan Kim wrote:
> PG_readahead flag is shared with PG_reclaim but PG_reclaim is only
> used in write context while PG_readahead is used for read context.
> 
> To make it clear, let's introduce PageReadahead wrapper with
> !PageWriteback so it could make code clear and we could drop
> PageWriteback check in page_cache_async_readahead, which removes
> pointless dropping mmap_sem.
> 
> Signed-off-by: Minchan Kim <minchan@...nel.org>

...

> +/* Clear PG_readahead only if it's PG_readahead, not PG_reclaim */
> +static inline int TestClearPageReadahead(struct page *page)
> +{
> +	VM_BUG_ON_PGFLAGS(PageCompound(page), page);
> +
> +	return !PageWriteback(page) ||
> +			test_and_clear_bit(PG_reclaim, &page->flags);
> +}

I think this is still wrong - if PageWriteback is not set, it will never
clear PG_reclaim bit so effectively the page will stay in PageReadahead
state!

The logic you really want to implement is:

	if (PageReadahead(page)) { <- this is your new PageReadahead
				    implementation
		clear_bit(PG_reclaim, &page->flags);
		return 1;
	}
	return 0;

Now this has the problem that it is not atomic. The only way I see to make
this fully atomic is using cmpxchg(). If we wanted to make this kinda-sorta
OK, the proper condition would look like:

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

Which is similar to what you originally had but different because in C '&&'
operator is not commutative due to side-effects committed at sequence points.

BTW: I share Andrew's view that we are piling hacks to fix problems caused
by older hacks. But I don't see any good option how to unalias
PG_readahead and PG_reclaim.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR