kernel/vm: Shrink commitments in discard_area_range if possible.

Confirmed by X512 to work with "mimalloc".

Change-Id: I981d6ef2d035a98f50b1b5cae1f698b9531e7dde
Reviewed-on: https://review.haiku-os.org/c/haiku/+/8682
Tested-by: Commit checker robot <[email protected]>
Haiku-Format: Haiku-format Bot <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/system/kernel/vm/vm.cpp

@@ -1120,12 +1120,31 @@ discard_area_range(VMArea* area, addr_t address, addr_t size)
 
 	unmap_pages(area, address, size);
 
+	ssize_t commitmentChange = 0;
+	if (cache->temporary && !cache->CanOvercommit() && area->page_protections != NULL) {
+		// See if the commitment can be shrunken after the pages are discarded.
+		const off_t areaCacheBase = area->Base() - area->cache_offset;
+		const off_t endAddress = address + size;
+		for (off_t pageAddress = address; pageAddress < endAddress; pageAddress += B_PAGE_SIZE) {
+			if (cache->LookupPage(pageAddress - areaCacheBase) == NULL)
+				continue;
+
+			const bool isWritable
+				= (get_area_page_protection(area, pageAddress) & B_WRITE_AREA) != 0;
+			if (!isWritable)
+				commitmentChange -= B_PAGE_SIZE;
+		}
+	}
+
 	// Since VMCache::Discard() can temporarily drop the lock, we must
 	// unlock all lower caches to prevent locking order inversion.
 	cacheChainLocker.Unlock(cache);
 	cache->Discard(cache->virtual_base + offset, size);
-	cache->ReleaseRefAndUnlock();
 
+	if (commitmentChange != 0)
+		cache->Commit(cache->committed_size + commitmentChange, VM_PRIORITY_USER);
+
+	cache->ReleaseRefAndUnlock();
 	return B_OK;
 }