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

John Stultz
2012-02-17T03:43:59Z

On Sun, 2012-02-12 at 13:48 +0100, Dmitry Adamushko wrote:
>
> On 10 February 2012 01:16, John Stultz <john.stultz@linaro.org> wrote:

> +static inline void volatile_range_shrink(struct
> volatile_range *range,
> + pgoff_t start_index, pgoff_t
> end_index)
> +{
> + size_t pre = range_size(range);
> +
> + range->range_node.start = start_index;
> + range->range_node.end = end_index;
> +
>
> I guess, here we get a whole range of races with volatile_shrink(),
> which may see inconsistent (in-the-middle-of-update) ranges
> (e.g. .start and .end).

We should be holding the vlist_mutex to avoid any such races. But you
also make clear that volatile_range_shrink() should really be called
volatile_range_resize(), since having two _shrink calls is terrible. My
apologies.

> + unsigned long nr_to_scan = sc->nr_to_scan;
> + const gfp_t gfp_mask = sc->gfp_mask;
> +
> + /* We might recurse into filesystem code, so bail out
> if necessary */
> + if (nr_to_scan && !(gfp_mask & __GFP_FS))
> + return -1;
> + if (!nr_to_scan)
> + return lru_count;
>
> So it's u64 -> int here, which is possibly 32 bits and signed. Can't
> it lead to inconsistent results on 32bit platforms?

Good point. Thanks for pointing that out.

> + start = range->range_node.start * PAGE_SIZE;
> + end = (range->range_node.end + 1) * PAGE_SIZE
> - 1;
>
> PAGE_CACHE_SHIFT was used in fadvise() to calculate .start and .end
> indexes, and here we use PAGE_SIZE to get back to 'normal' addresses.
> Isn't it inconsistent at the very least?

Fair enough.

>
> + nr_to_scan -= range_size(range);
>
> hmm, unsigned long -= u64
>
> + if (nr_to_scan <= 0)
>
> nr_to_scan is "unsigned long" :-))

Good catch.

Thanks for the feedback!
-john