On Tue, 21 Jul 2015 11:09:35 -0700 Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> fallocate hole punch will want to remove a specific range of
> pages. When pages are removed, their associated entries in
> the region/reserve map will also be removed. This will break
> an assumption in the region_chg/region_add calling sequence.
> If a new region descriptor must be allocated, it is done as
> part of the region_chg processing. In this way, region_add
> can not fail because it does not need to attempt an allocation.
>
> To prepare for fallocate hole punch, create a "cache" of
> descriptors that can be used by region_add if necessary.
> region_chg will ensure there are sufficient entries in the
> cache. It will be necessary to track the number of in progress
> add operations to know a sufficient number of descriptors
> reside in the cache. A new routine region_abort is added to
> adjust this in progress count when add operations are aborted.
> vma_abort_reservation is also added for callers creating
> reservations with vma_needs_reservation/vma_commit_reservation.
>
> ...
>
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -35,6 +35,9 @@ struct resv_map {
> struct kref refs;
> spinlock_t lock;
> struct list_head regions;
> + long adds_in_progress;
> + struct list_head rgn_cache;
> + long rgn_cache_count;
> };
Linux style is to spell words out fully: rgn->region. One advantage of
doing this is that we get consistency: when there's no doubt about how
a thing is spelled we don't end up with various different terms for the
same thing. Note that we already have resv_map.regions and struct
file_region.
To avoid a respin I think I'll just do a s/rgn/region/g on the patches
then take a look for 80 col overflow - shout at me if you dislike that.
And yes, resv_map should have been called reservation_map. I suck.
And there's region_chg. Who wrote this stuff.
As an off-topic cleanup, resv_map and its associated stuff could be
moved out of include/linux/hugetlb.h and made private to hugetlb.c with
some minor changes.
Finally, the data structure definition site is a great place at which
to document the overall design. What the fields do, how they interact,
locking design, etc. eg, what data structure is at
resv_map.region_cache, how is it managed, what purpose does it serve
etc.
And adds_in_progress is interesting. I see the comment over
region_abort() (rgn_abort?!) but haven't quite soaked that in yet.
>
> ...
>
> @@ -312,11 +339,16 @@ static long region_add(struct resv_map *resv, long f, long t)
> * so that the subsequent region_add call will have all the
> * regions it needs and will not fail.
> *
> + * Upon entry, region_chg will also examine the cache of
> + * region descriptors associated with the map. If there
"are"
> + * not enough descriptors cached, one will be allocated
> + * for the in progress add operation.
> + *
> * Returns the number of huge pages that need to be added
> * to the existing reservation map for the range [f, t).
> * This number is greater or equal to zero. -ENOMEM is
> - * returned if a new file_region structure is needed and can
> - * not be allocated.
> + * returned if a new file_region structure or cache entry
> + * is needed and can not be allocated.
> */
> static long region_chg(struct resv_map *resv, long f, long t)
> {
>
> ...
>
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