uprobes: turn xol_area->pages[2] into xol_area->page

Now that xol_mapping has its own ->fault() method we no longer need
xol_area->pages[1] == NULL, we need a single page.

Link: https://lkml.kernel.org/r/20240911131437.GC3448@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: oleg-nesterov

kernel/events/uprobes.c

@@ -99,7 +99,7 @@ struct xol_area {
 	atomic_t 			slot_count;	/* number of in-use slots */
 	unsigned long 			*bitmap;	/* 0 = free slot */
 
-	struct page 			*pages[2];
+	struct page			*page;
 	/*
 	 * We keep the vma's vm_start rather than a pointer to the vma
 	 * itself.  The probed process or a naughty kernel module could make
@@ -1437,7 +1437,7 @@ static vm_fault_t xol_fault(const struct vm_special_mapping *sm,
 {
 	struct xol_area *area = vma->vm_mm->uprobes_state.xol_area;
 
-	vmf->page = area->pages[0];
+	vmf->page = area->page;
 	get_page(vmf->page);
 	return 0;
 }
@@ -1512,23 +1512,22 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
 	if (!area->bitmap)
 		goto free_area;
 
-	area->pages[0] = alloc_page(GFP_HIGHUSER);
-	if (!area->pages[0])
+	area->page = alloc_page(GFP_HIGHUSER);
+	if (!area->page)
 		goto free_bitmap;
-	area->pages[1] = NULL;
 
 	area->vaddr = vaddr;
 	init_waitqueue_head(&area->wq);
 	/* Reserve the 1st slot for get_trampoline_vaddr() */
 	set_bit(0, area->bitmap);
 	atomic_set(&area->slot_count, 1);
 	insns = arch_uprobe_trampoline(&insns_size);
-	arch_uprobe_copy_ixol(area->pages[0], 0, insns, insns_size);
+	arch_uprobe_copy_ixol(area->page, 0, insns, insns_size);
 
 	if (!xol_add_vma(mm, area))
 		return area;
 
-	__free_page(area->pages[0]);
+	__free_page(area->page);
  free_bitmap:
 	kfree(area->bitmap);
  free_area:
@@ -1570,7 +1569,7 @@ void uprobe_clear_state(struct mm_struct *mm)
 	if (!area)
 		return;
 
-	put_page(area->pages[0]);
+	put_page(area->page);
 	kfree(area->bitmap);
 	kfree(area);
 }
@@ -1637,7 +1636,7 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
 	if (unlikely(!xol_vaddr))
 		return 0;
 
-	arch_uprobe_copy_ixol(area->pages[0], xol_vaddr,
+	arch_uprobe_copy_ixol(area->page, xol_vaddr,
 			      &uprobe->arch.ixol, sizeof(uprobe->arch.ixol));
 
 	return xol_vaddr;