Re: [PATCH 2/2] [RFC] fadvise: Add _VOLATILE,_ISVOLATILE, and _NONVOLATILEflags

[John Stultz]

On 03/17/2012 09:21 AM, Dmitry Adamushko wrote:
> 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().
Good catch, thank you!

> 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).
I agree that this seems like a much more intelligent way coalesce 
regions.   I hadn't yet implemented it, as I was hoping for some comment 
from the Android folks if there was a specific use for the design they 
selected for ashmem, but I suspect there isn't.

I'll go ahead and integrate this for the next revision.

Thanks again for the feedback!
-john

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