From: Jan Kara <jack@suse.cz>
To: Minchan Kim <minchan@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
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 3/3] mm: make PageReadahead more strict
Date: Mon, 17 Feb 2020 10:31:28 +0100 [thread overview]
Message-ID: <20200217093128.GB12032@quack2.suse.cz> (raw)
In-Reply-To: <20200212221614.215302-3-minchan@kernel.org>
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@kernel.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@suse.com>
SUSE Labs, CR
next prev parent reply other threads:[~2020-02-17 9:31 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-12 22:16 [PATCH 1/3] mm: Don't bother dropping mmap_sem for zero size readahead Minchan Kim
2020-02-12 22:16 ` [PATCH 2/3] mm: fix long time stall from mm_populate Minchan Kim
2020-04-21 4:12 ` Andrew Morton
2020-02-12 22:16 ` [PATCH 3/3] mm: make PageReadahead more strict Minchan Kim
2020-02-17 9:31 ` Jan Kara [this message]
2020-02-18 22:28 ` Minchan Kim
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