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From: Christoph Hellwig <hch@infradead.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10 08/33] mm: Add folio_try_get_rcu
Date: Thu, 27 May 2021 09:16:42 +0100	[thread overview]
Message-ID: <YK9VagEO+bKurYlz@infradead.org> (raw)
In-Reply-To: <20210511214735.1836149-9-willy@infradead.org>

On Tue, May 11, 2021 at 10:47:10PM +0100, Matthew Wilcox (Oracle) wrote:
> -static inline int page_ref_add_unless(struct page *page, int nr, int u)
> +static inline bool page_ref_add_unless(struct page *page, int nr, int u)
>  {
> -	int ret = atomic_add_unless(&page->_refcount, nr, u);
> +	bool ret = atomic_add_unless(&page->_refcount, nr, u);
>  
>  	if (page_ref_tracepoint_active(page_ref_mod_unless))
>  		__page_ref_mod_unless(page, nr, ret);
>  	return ret;
>  }

Unrelated but neat cleanup.

>  
> -static inline int folio_ref_add_unless(struct folio *folio, int nr, int u)
> +static inline bool folio_ref_add_unless(struct folio *folio, int nr, int u)
>  {
>  	return page_ref_add_unless(&folio->page, nr, u);
>  }

This should probably go into the patch adding folio_ref_add_unless.

> +static inline bool folio_ref_try_add_rcu(struct folio *folio, int count)

Should this have a __ prefix and/or a don't use direct comment?

> +{
> +#ifdef CONFIG_TINY_RCU
> +	/*
> +	 * The caller guarantees the folio will not be freed from interrupt
> +	 * context, so (on !SMP) we only need preemption to be disabled
> +	 * and TINY_RCU does that for us.
> +	 */
> +# ifdef CONFIG_PREEMPT_COUNT
> +	VM_BUG_ON(!in_atomic() && !irqs_disabled());
> +# endif

	VM_BUG_ON(IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
		  !in_atomic() && !irqs_disabled());

?

> +	VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio);
> +	folio_ref_add(folio, count);
> +#else
> +	if (unlikely(!folio_ref_add_unless(folio, count, 0))) {
> +		/* Either the folio has been freed, or will be freed. */
> +		return false;
> +	}
> +#endif
> +	return true;

but is this tiny rcu optimization really worth it?  I guess we're just
preserving it from the existing code and don't rock the boat..

> @@ -1746,6 +1746,26 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
>  }
>  EXPORT_SYMBOL(page_cache_prev_miss);
>  
> +/*
> + * Lockless page cache protocol:
> + * On the lookup side:
> + * 1. Load the folio from i_pages
> + * 2. Increment the refcount if it's not zero
> + * 3. If the folio is not found by xas_reload(), put the refcount and retry
> + *
> + * On the removal side:
> + * A. Freeze the page (by zeroing the refcount if nobody else has a reference)
> + * B. Remove the page from i_pages
> + * C. Return the page to the page allocator
> + *
> + * This means that any page may have its reference count temporarily
> + * increased by a speculative page cache (or fast GUP) lookup as it can
> + * be allocated by another user before the RCU grace period expires.
> + * Because the refcount temporarily acquired here may end up being the
> + * last refcount on the page, any page allocation must be freeable by
> + * put_folio().
> + */
> +
>  /*
>   * mapping_get_entry - Get a page cache entry.
>   * @mapping: the address_space to search

Is this really a good place for the comment?  I'd expect it either near
a relevant function or at the top of a file.

  parent reply	other threads:[~2021-05-27  8:17 UTC|newest]

Thread overview: 96+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-11 21:47 [PATCH v10 00/33] Memory folios Matthew Wilcox (Oracle)
2021-05-11 21:47 ` [PATCH v10 01/33] mm: Introduce struct folio Matthew Wilcox (Oracle)
2021-05-14 10:34   ` Vlastimil Babka
2021-05-14 10:40   ` Vlastimil Babka
2021-05-14 11:47     ` Matthew Wilcox
2021-05-15 10:55   ` William Kucharski
2021-05-15 20:14     ` Matthew Wilcox
2021-05-16 19:26       ` William Kucharski
2021-05-27  8:09   ` Christoph Hellwig
2021-05-11 21:47 ` [PATCH v10 02/33] mm: Add folio_pgdat and folio_zone Matthew Wilcox (Oracle)
2021-05-14 10:35   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 03/33] mm/vmstat: Add functions to account folio statistics Matthew Wilcox (Oracle)
2021-05-14 10:36   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 04/33] mm/debug: Add VM_BUG_ON_FOLIO and VM_WARN_ON_ONCE_FOLIO Matthew Wilcox (Oracle)
2021-05-14 10:44   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 05/33] mm: Add folio reference count functions Matthew Wilcox (Oracle)
2021-05-14 11:04   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 06/33] mm: Add folio_put Matthew Wilcox (Oracle)
2021-05-14 11:52   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 07/33] mm: Add folio_get Matthew Wilcox (Oracle)
2021-05-14 11:56   ` Vlastimil Babka
2021-05-14 14:24     ` Matthew Wilcox
2021-05-14 15:39       ` Vlastimil Babka
2021-05-27  8:10       ` Christoph Hellwig
2021-05-27 22:53         ` Andrew Morton
2021-05-11 21:47 ` [PATCH v10 08/33] mm: Add folio_try_get_rcu Matthew Wilcox (Oracle)
2021-05-14 12:11   ` Vlastimil Babka
2021-05-27  8:16   ` Christoph Hellwig [this message]
2021-06-05  4:26     ` Matthew Wilcox
2021-06-06 14:13       ` Christoph Hellwig
2021-05-11 21:47 ` [PATCH v10 09/33] mm: Add folio flag manipulation functions Matthew Wilcox (Oracle)
2021-05-14 15:29   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 10/33] mm: Add folio_young and folio_idle Matthew Wilcox (Oracle)
2021-05-14 15:33   ` Vlastimil Babka
2021-05-27  8:17   ` Christoph Hellwig
2021-05-11 21:47 ` [PATCH v10 11/33] mm: Handle per-folio private data Matthew Wilcox (Oracle)
2021-05-14 15:41   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 12/33] mm/filemap: Add folio_index, folio_file_page and folio_contains Matthew Wilcox (Oracle)
2021-05-14 15:55   ` Vlastimil Babka
2021-05-15 15:51     ` Matthew Wilcox
2021-05-11 21:47 ` [PATCH v10 13/33] mm/filemap: Add folio_next_index Matthew Wilcox (Oracle)
2021-05-14 17:07   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 14/33] mm/filemap: Add folio_offset and folio_file_offset Matthew Wilcox (Oracle)
2021-05-14 17:08   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 15/33] mm/util: Add folio_mapping and folio_file_mapping Matthew Wilcox (Oracle)
2021-05-14 17:29   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 16/33] mm: Add folio_mapcount Matthew Wilcox (Oracle)
2021-05-14 17:39   ` Vlastimil Babka
2021-05-18 18:45   ` Matthew Wilcox
2021-05-11 21:47 ` [PATCH v10 17/33] mm/memcg: Add folio wrappers for various functions Matthew Wilcox (Oracle)
2021-05-18  9:57   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 18/33] mm/filemap: Add folio_unlock Matthew Wilcox (Oracle)
2021-05-18 10:06   ` Vlastimil Babka
2021-05-18 11:30     ` Matthew Wilcox
2021-05-11 21:47 ` [PATCH v10 19/33] mm/filemap: Add folio_lock Matthew Wilcox (Oracle)
2021-05-18 10:26   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 20/33] mm/filemap: Add folio_lock_killable Matthew Wilcox (Oracle)
2021-05-18 10:31   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 21/33] mm/filemap: Add __folio_lock_async Matthew Wilcox (Oracle)
2021-05-18 10:34   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 22/33] mm/filemap: Add __folio_lock_or_retry Matthew Wilcox (Oracle)
2021-05-18 10:38   ` Vlastimil Babka
2021-05-18 10:45     ` Vlastimil Babka
2021-05-18 13:35     ` Matthew Wilcox
2021-05-11 21:47 ` [PATCH v10 23/33] mm/filemap: Add folio_wait_locked Matthew Wilcox (Oracle)
2021-05-18 10:41   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 24/33] mm/swap: Add folio_rotate_reclaimable Matthew Wilcox (Oracle)
2021-05-18 10:48   ` Vlastimil Babka
2021-05-27  8:19   ` Christoph Hellwig
2021-05-11 21:47 ` [PATCH v10 25/33] mm/filemap: Add folio_end_writeback Matthew Wilcox (Oracle)
2021-05-18 11:08   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 26/33] mm/writeback: Add folio_wait_writeback Matthew Wilcox (Oracle)
2021-05-18 11:12   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 27/33] mm/writeback: Add folio_wait_stable Matthew Wilcox (Oracle)
2021-05-18 11:42   ` Vlastimil Babka
2021-05-18 13:55     ` Matthew Wilcox
2021-05-11 21:47 ` [PATCH v10 28/33] mm/filemap: Add folio_wait_bit Matthew Wilcox (Oracle)
2021-05-18 11:51   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 29/33] mm/filemap: Add folio_wake_bit Matthew Wilcox (Oracle)
2021-05-18 11:53   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 30/33] mm/filemap: Convert page wait queues to be folios Matthew Wilcox (Oracle)
2021-05-18 12:23   ` Vlastimil Babka
2021-05-11 21:47 ` [PATCH v10 31/33] mm/filemap: Add folio private_2 functions Matthew Wilcox (Oracle)
2021-05-18 12:26   ` Vlastimil Babka
2021-05-27  8:21   ` Christoph Hellwig
2021-05-11 21:47 ` [PATCH v10 32/33] fs/netfs: Add folio fscache functions Matthew Wilcox (Oracle)
2021-05-18 13:48   ` Vlastimil Babka
2021-05-27  8:23   ` Christoph Hellwig
2021-05-11 21:47 ` [PATCH v10 33/33] mm: Add folio_mapped Matthew Wilcox (Oracle)
2021-05-18 14:17   ` Vlastimil Babka
2021-05-27  8:31   ` Christoph Hellwig
2021-05-13 14:50 ` [PATCH v10 00/33] Memory folios Matthew Wilcox
2021-05-15 10:26 ` William Kucharski
2021-06-04  1:07 ` Matteo Croce
2021-06-04  2:13   ` Matthew Wilcox
2021-06-08 14:56     ` Matteo Croce

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