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Subject[RFC v1] mm: add the preempt check into alloc_vmap_area()
Date
During finding a suitable hole in the vmap_area_list
there is an explicit rescheduling check for latency reduction.
We do it, since there are workloads which are sensitive for
long (more than 1 millisecond) preemption off scenario.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
mm/vmalloc.c | 57 +++++++++++++++++++++++++++++++++++++++++++++------------
1 file changed, 45 insertions(+), 12 deletions(-)

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 673942094328..60a57752f8fc 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -325,6 +325,7 @@ EXPORT_SYMBOL(vmalloc_to_pfn);

#define VM_LAZY_FREE 0x02
#define VM_VM_AREA 0x04
+#define VM_LAZY_FREE_DEFER 0x08

static DEFINE_SPINLOCK(vmap_area_lock);
/* Export for kexec only */
@@ -491,6 +492,20 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
if (addr + size < addr)
goto overflow;

+ /*
+ * Put on hold this VA preventing it from being
+ * removed from the list because of dropping the
+ * vmap_area_lock. It means we are save to proceed
+ * the search after the lock is taken again if we
+ * were scheduled out or the spin needed a break.
+ */
+ if (gfpflags_allow_blocking(gfp_mask) &&
+ !(first->flags & VM_LAZY_FREE_DEFER)) {
+ first->flags |= VM_LAZY_FREE_DEFER;
+ cond_resched_lock(&vmap_area_lock);
+ first->flags &= ~VM_LAZY_FREE_DEFER;
+ }
+
if (list_is_last(&first->list, &vmap_area_list))
goto found;

@@ -586,16 +601,6 @@ static void __free_vmap_area(struct vmap_area *va)
}

/*
- * Free a region of KVA allocated by alloc_vmap_area
- */
-static void free_vmap_area(struct vmap_area *va)
-{
- spin_lock(&vmap_area_lock);
- __free_vmap_area(va);
- spin_unlock(&vmap_area_lock);
-}
-
-/*
* Clear the pagetable entries of a given vmap_area
*/
static void unmap_vmap_area(struct vmap_area *va)
@@ -678,6 +683,7 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
struct vmap_area *va;
struct vmap_area *n_va;
bool do_free = false;
+ int va_nr_pages;

lockdep_assert_held(&vmap_purge_lock);

@@ -697,10 +703,19 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)

spin_lock(&vmap_area_lock);
llist_for_each_entry_safe(va, n_va, valist, purge_list) {
- int nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
+ if (unlikely(va->flags & VM_LAZY_FREE_DEFER)) {
+ /*
+ * Put deferred VA back to the vmap_purge_list.
+ * We do not need to modify vmap_lazy_nr since
+ * the va will not be removed now.
+ */
+ llist_add(&va->purge_list, &vmap_purge_list);
+ continue;
+ }

+ va_nr_pages = (va->va_end - va->va_start) >> PAGE_SHIFT;
__free_vmap_area(va);
- atomic_sub(nr, &vmap_lazy_nr);
+ atomic_sub(va_nr_pages, &vmap_lazy_nr);
cond_resched_lock(&vmap_area_lock);
}
spin_unlock(&vmap_area_lock);
@@ -750,6 +765,24 @@ static void free_vmap_area_noflush(struct vmap_area *va)
}

/*
+ * Free a region of KVA allocated by alloc_vmap_area
+ */
+static void free_vmap_area(struct vmap_area *va)
+{
+ bool do_lazy_free = false;
+
+ spin_lock(&vmap_area_lock);
+ if (unlikely(va->flags & VM_LAZY_FREE_DEFER))
+ do_lazy_free = true;
+ else
+ __free_vmap_area(va);
+ spin_unlock(&vmap_area_lock);
+
+ if (unlikely(do_lazy_free))
+ free_vmap_area_noflush(va);
+}
+
+/*
* Free and unmap a vmap area
*/
static void free_unmap_vmap_area(struct vmap_area *va)
--
2.11.0
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