[RFC PATCH 1/3] mm: hugetlb: Simplify logic in dequeue_hugetlb_folio_vma()

[Ackerley Tng <ackerleytng@google.com>]

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 190fa05635f4..73165c670739 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1281,8 +1281,9 @@ 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)) {
 		/*
@@ -1394,8 +1395,7 @@ 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;
@@ -1403,16 +1403,7 @@ 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);
@@ -1430,7 +1421,7 @@ 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--;
 	}
@@ -2973,6 +2964,7 @@ 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);
@@ -3033,20 +3025,17 @@ 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--;
 		}
--
2.47.0.rc1.288.g06298d1525-goog