linux-kernel - Re: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and

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Date:	Sat, 17 Mar 2012 17:21:48 +0100
From:	Dmitry Adamushko <dmitry.adamushko@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>, Neil Brown <neilb@...e.de>,
	Andrea Righi <andrea@...terlinux.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and
 _NONVOLATILE flags

Hi John,

[ ... ]

> +/*
> + * Mark a region as volatile, allowing dirty pages to be purged
> + * under memory pressure
> + */
> +long mapping_range_volatile(struct address_space *mapping,
> +                               pgoff_t start_index, pgoff_t end_index)
> +{
> +       struct volatile_range *new;
> +       struct range_tree_node *node;
> +
> +       u64 start, end;
> +       int purged = 0;
> +       start = (u64)start_index;
> +       end = (u64)end_index;
> +
> +       new = vrange_alloc();
> +       if (!new)
> +               return -ENOMEM;
> +
> +       mutex_lock(&volatile_mutex);
> +
> +       node = range_tree_in_range_adjacent(&mapping->volatile_root,
> +                                               start, end);
> +       while (node) {
> +               struct volatile_range *vrange;
> +
> +               /* Already entirely marked volatile, so we're done */
> +               if (node->start < start && node->end > end) {
> +                       /* don't need the allocated value */
> +                       kfree(new);
> +                       goto out;
> +               }
> +
> +               /* Grab containing volatile range */
> +               vrange = container_of(node, struct volatile_range, range_node);
> +
> +               /* resize range */
> +               start = min_t(u64, start, node->start);
> +               end = max_t(u64, end, node->end);
> +               purged |= vrange->purged;
> +
> +
> +               vrange_del(vrange);
> +
> +               /* get the next possible overlap */
> +               node = range_tree_in_range(&mapping->volatile_root, start, end);

I guess range_tree_in_range_adjacent() should be used here again.
There can be 2 adjacent regions (left and right), and we'll miss one
of them with range_tree_in_range().

Also (as I had already mentioned before), I think that new ranges must
not be merged with the existing "vrange->purged == 1" ranges.
Otherwise, for some use cases, the whole idea of 'volatility' gets
broken. For example, when an application is processing a big buffer in
small consequent chunks (marking a chunk as volatile when done with
it), and the range gets 'purged' by the kernel early in this process
(when it's still small).

-- Dmitry
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