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SubjectRe: [PATCH v5 4/7] hugetlb: disable region_add file_region coalescing
On Fri, Sep 27, 2019 at 2:44 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 9/19/19 3:24 PM, Mina Almasry wrote:
> > A follow up patch in this series adds hugetlb cgroup uncharge info the
> > file_region entries in resv->regions. The cgroup uncharge info may
> > differ for different regions, so they can no longer be coalesced at
> > region_add time. So, disable region coalescing in region_add in this
> > patch.
> >
> > Behavior change:
> >
> > Say a resv_map exists like this [0->1], [2->3], and [5->6].
> >
> > Then a region_chg/add call comes in region_chg/add(f=0, t=5).
> >
> > Old code would generate resv->regions: [0->5], [5->6].
> > New code would generate resv->regions: [0->1], [1->2], [2->3], [3->5],
> > [5->6].
> >
> > Special care needs to be taken to handle the resv->adds_in_progress
> > variable correctly. In the past, only 1 region would be added for every
> > region_chg and region_add call. But now, each call may add multiple
> > regions, so we can no longer increment adds_in_progress by 1 in region_chg,
> > or decrement adds_in_progress by 1 after region_add or region_abort. Instead,
> > region_chg calls add_reservation_in_range() to count the number of regions
> > needed and allocates those, and that info is passed to region_add and
> > region_abort to decrement adds_in_progress correctly.
> >
> > Signed-off-by: Mina Almasry <almasrymina@google.com>
> >
> > ---
> > mm/hugetlb.c | 273 +++++++++++++++++++++++++++++----------------------
> > 1 file changed, 158 insertions(+), 115 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index bac1cbdd027c..d03b048084a3 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -244,6 +244,12 @@ struct file_region {
> > long to;
> > };
> >
> > +/* Helper that removes a struct file_region from the resv_map cache and returns
> > + * it for use.
> > + */
> > +static struct file_region *
> > +get_file_region_entry_from_cache(struct resv_map *resv, long from, long to);
> > +
>
> Instead of the forward declaration, just put the function here.
>
> > /* Must be called with resv->lock held. Calling this with count_only == true
> > * will count the number of pages to be added but will not modify the linked
> > * list.
> > @@ -251,51 +257,61 @@ struct file_region {
> > static long add_reservation_in_range(struct resv_map *resv, long f, long t,
> > bool count_only)
> > {
> > - long chg = 0;
> > + long add = 0;
> > struct list_head *head = &resv->regions;
> > + long last_accounted_offset = f;
> > struct file_region *rg = NULL, *trg = NULL, *nrg = NULL;
> >
> > - /* Locate the region we are before or in. */
> > - list_for_each_entry (rg, head, link)
> > - if (f <= rg->to)
> > - break;
> > -
> > - /* Round our left edge to the current segment if it encloses us. */
> > - if (f > rg->from)
> > - f = rg->from;
> > -
> > - chg = t - f;
> > + /* In this loop, we essentially handle an entry for the range
> > + * last_accounted_offset -> rg->from, at every iteration, with some
> > + * bounds checking.
> > + */
> > + list_for_each_entry_safe(rg, trg, head, link) {
> > + /* Skip irrelevant regions that start before our range. */
> > + if (rg->from < f) {
> > + /* If this region ends after the last accounted offset,
> > + * then we need to update last_accounted_offset.
> > + */
> > + if (rg->to > last_accounted_offset)
> > + last_accounted_offset = rg->to;
> > + continue;
> > + }
> >
> > - /* Check for and consume any regions we now overlap with. */
> > - nrg = rg;
> > - list_for_each_entry_safe (rg, trg, rg->link.prev, link) {
> > - if (&rg->link == head)
> > - break;
> > + /* When we find a region that starts beyond our range, we've
> > + * finished.
> > + */
> > if (rg->from > t)
> > break;
> >
> > - /* We overlap with this area, if it extends further than
> > - * us then we must extend ourselves. Account for its
> > - * existing reservation.
> > + /* Add an entry for last_accounted_offset -> rg->from, and
> > + * update last_accounted_offset.
> > */
> > - if (rg->to > t) {
> > - chg += rg->to - t;
> > - t = rg->to;
> > + if (rg->from > last_accounted_offset) {
> > + add += rg->from - last_accounted_offset;
> > + if (!count_only) {
> > + nrg = get_file_region_entry_from_cache(
> > + resv, last_accounted_offset, rg->from);
> > + list_add(&nrg->link, rg->link.prev);
> > + }
> > }
> > - chg -= rg->to - rg->from;
> >
> > - if (!count_only && rg != nrg) {
> > - list_del(&rg->link);
> > - kfree(rg);
> > - }
> > + last_accounted_offset = rg->to;
> > }
> >
> > - if (!count_only) {
> > - nrg->from = f;
> > - nrg->to = t;
> > + /* Handle the case where our range extends beyond
> > + * last_accounted_offset.
> > + */
> > + if (last_accounted_offset < t) {
> > + add += t - last_accounted_offset;
> > + if (!count_only) {
> > + nrg = get_file_region_entry_from_cache(
> > + resv, last_accounted_offset, t);
> > + list_add(&nrg->link, rg->link.prev);
> > + }
> > + last_accounted_offset = t;
> > }
> >
> > - return chg;
> > + return add;
> > }
> >
> > /*
> > @@ -305,46 +321,24 @@ static long add_reservation_in_range(struct resv_map *resv, long f, long t,
>
> The start of this comment block says,
>
> /*
> * Add the huge page range represented by [f, t) to the reserve
> * map. Existing regions will be expanded to accommodate the specified
> * range, or a region will be taken from the cache.
>
> We are no longer expanding existing regions. Correct?
> As an optimization, I guess we could coalesce/combine reion entries as
> long as they are for the same cgroup. However, it may not be worth the
> effort.
>
> > * must exist in the cache due to the previous call to region_chg with
> > * the same range.
> > *
> > + * regions_needed is the out value provided by a previous
> > + * call to region_chg.
> > + *
> > * Return the number of new huge pages added to the map. This
> > * number is greater than or equal to zero.
> > */
> > -static long region_add(struct resv_map *resv, long f, long t)
> > +static long region_add(struct resv_map *resv, long f, long t,
> > + long regions_needed)
> > {
> > - struct list_head *head = &resv->regions;
> > - struct file_region *rg, *nrg;
> > long add = 0;
> >
> > spin_lock(&resv->lock);
> > - /* Locate the region we are either in or before. */
> > - list_for_each_entry(rg, head, link)
> > - if (f <= rg->to)
> > - break;
> >
> > - /*
> > - * If no region exists which can be expanded to include the
> > - * specified range, pull a region descriptor from the cache
> > - * and use it for this range.
> > - */
> > - if (&rg->link == head || t < rg->from) {
> > - VM_BUG_ON(resv->region_cache_count <= 0);
> > -
> > - resv->region_cache_count--;
> > - nrg = list_first_entry(&resv->region_cache, struct file_region,
> > - link);
> > - list_del(&nrg->link);
> > -
> > - nrg->from = f;
> > - nrg->to = t;
> > - list_add(&nrg->link, rg->link.prev);
> > -
> > - add += t - f;
> > - goto out_locked;
> > - }
> > + VM_BUG_ON(resv->region_cache_count < regions_needed);
> >
> > add = add_reservation_in_range(resv, f, t, false);
> > + resv->adds_in_progress -= regions_needed;
>
> Consider this example,
>
> - region_chg(1,2)
> adds_in_progress = 1
> cache entries 1
> - region_chg(3,4)
> adds_in_progress = 2
> cache entries 2
> - region_chg(5,6)
> adds_in_progress = 3
> cache entries 3
>
> At this point, no region descriptors are in the map because only
> region_chg has been called.
>
> - region_chg(0,6)
> adds_in_progress = 4
> cache entries 4
>
> Is that correct so far?
>
> Then the following sequence happens,
>
> - region_add(1,2)
> adds_in_progress = 3
> cache entries 3
> - region_add(3,4)
> adds_in_progress = 2
> cache entries 2
> - region_add(5,6)
> adds_in_progress = 1
> cache entries 1
>
> list of region descriptors is:
> [1->2] [3->4] [5->6]
>
> - region_add(0,6)
> This is going to require 3 cache entries but only one is in the cache.
> I think we are going to BUG in get_file_region_entry_from_cache() the
> second time it is called from add_reservation_in_range().
>
> I stopped looking at the code here as things will need to change if this
> is a real issue.

You're right. I had been assuming that some higher level sync causes
to ban a sequence of region_chg calls without a region_add or
region_abort to resolve each region_chg call, but seems that is not
the case. I'll fix and upload another version of the patch.

> --
> Mike Kravetz

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