[RFC PATCH 01/39] mm: hugetlb: Simplify logic in dequeue_hugetlb_folio_vma()

Ackerley Tng
2024-09-10T23:44:35Z

Replace arguments avoid_reserve and chg in dequeue_hugetlb_folio_vma()
so dequeue_hugetlb_folio_vma() is more understandable.

The new argument, use_hstate_resv, indicates whether the folio to be
dequeued should be taken from reservations in hstate.

If use_hstate_resv is true, the folio to be dequeued should be taken
from reservations in hstate and hence h->resv_huge_pages is
decremented, and the folio is marked so that the reservation is
restored.

If use_hstate_resv is false, then a folio needs to be taken from the
pool and hence there must exist available_huge_pages(h), failing
which, goto err.

The bool use_hstate_resv can be reused within
dequeue_hugetlb_folio_vma()'s caller, alloc_hugetlb_folio().

No functional changes are intended.

As proof, the original two if conditions

!vma_has_reserves(vma, chg) && !available_huge_pages(h)

and

avoid_reserve && !available_huge_pages(h)

can be combined into

(avoid_reserve || !vma_has_reserves(vma, chg))
&& !available_huge_pages(h).

Applying de Morgan's theorem on

avoid_reserve || !vma_has_reserves(vma, chg)

yields

!avoid_reserve && vma_has_reserves(vma, chg),

hence the simplification is correct.

Signed-off-by: Ackerley Tng <ackerleytng@google.com>
---
mm/hugetlb.c | 33 +++++++++++----------------------
1 file changed, 11 insertions(+), 22 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index aaf508be0a2b..af5c6bbc9ff0 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1280,8 +1280,9 @@ static bool vma_has_reserves(struct vm_area_struct *vma, long chg)
}

/*
- * Only the process that called mmap() has reserves for
- * private mappings.
+ * Only the process that called mmap() has reserves for private
+ * mappings. A child process with MAP_PRIVATE mappings created by their
+ * parent have no page reserves.
*/
if (is_vma_resv_set(vma, HPAGE_RESV_OWNER)) {
/*
@@ -1393,8 +1394,7 @@ static unsigned long available_huge_pages(struct hstate *h)

static struct folio *dequeue_hugetlb_folio_vma(struct hstate *h,
struct vm_area_struct *vma,
- unsigned long address, int avoid_reserve,
- long chg)
+ unsigned long address, bool use_hstate_resv)
{
struct folio *folio = NULL;
struct mempolicy *mpol;
@@ -1402,16 +1402,7 @@ static struct folio *dequeue_hugetlb_folio_vma(struct hstate *h,
nodemask_t *nodemask;
int nid;

- /*
- * A child process with MAP_PRIVATE mappings created by their parent
- * have no page reserves. This check ensures that reservations are
- * not "stolen". The child may still get SIGKILLed
- */
- if (!vma_has_reserves(vma, chg) && !available_huge_pages(h))
- goto err;
-
- /* If reserves cannot be used, ensure enough pages are in the pool */
- if (avoid_reserve && !available_huge_pages(h))
+ if (!use_hstate_resv && !available_huge_pages(h))
goto err;

gfp_mask = htlb_alloc_mask(h);
@@ -1429,7 +1420,7 @@ static struct folio *dequeue_hugetlb_folio_vma(struct hstate *h,
folio = dequeue_hugetlb_folio_nodemask(h, gfp_mask,
nid, nodemask);

- if (folio && !avoid_reserve && vma_has_reserves(vma, chg)) {
+ if (folio && use_hstate_resv) {
folio_set_hugetlb_restore_reserve(folio);
h->resv_huge_pages--;
}
@@ -3130,6 +3121,7 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
struct mem_cgroup *memcg;
bool deferred_reserve;
gfp_t gfp = htlb_alloc_mask(h) | __GFP_RETRY_MAYFAIL;
+ bool use_hstate_resv;

memcg = get_mem_cgroup_from_current();
memcg_charge_ret = mem_cgroup_hugetlb_try_charge(memcg, gfp, nr_pages);
@@ -3190,20 +3182,17 @@ struct folio *alloc_hugetlb_folio(struct vm_area_struct *vma,
if (ret)
goto out_uncharge_cgroup_reservation;

+ use_hstate_resv = !avoid_reserve && vma_has_reserves(vma, gbl_chg);
+
spin_lock_irq(&hugetlb_lock);
- /*
- * glb_chg is passed to indicate whether or not a page must be taken
- * from the global free pool (global change). gbl_chg == 0 indicates
- * a reservation exists for the allocation.
- */
- folio = dequeue_hugetlb_folio_vma(h, vma, addr, avoid_reserve, gbl_chg);
+ folio = dequeue_hugetlb_folio_vma(h, vma, addr, use_hstate_resv);
if (!folio) {
spin_unlock_irq(&hugetlb_lock);
folio = alloc_buddy_hugetlb_folio_with_mpol(h, vma, addr);
if (!folio)
goto out_uncharge_cgroup;
spin_lock_irq(&hugetlb_lock);
- if (!avoid_reserve && vma_has_reserves(vma, gbl_chg)) {
+ if (use_hstate_resv) {
folio_set_hugetlb_restore_reserve(folio);
h->resv_huge_pages--;
}
--
2.46.0.598.g6f2099f65c-goog