Re: [PATCH 1/1] page cache: fix page_cache_next/prev_miss off by one

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Mike Kravetz <mike.kravetz@xxxxxxxxxx> writes:

diff --git a/mm/filemap.c b/mm/filemap.c
index 71dc90f64e43..123540c7ba45 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1733,7 +1733,9 @@ bool __folio_lock_or_retry(struct folio *folio, struct mm_struct *mm,
   *
   * Return: The index of the gap if found, otherwise an index outside the
* range specified (in which case 'return - index >= max_scan' will be true).
- * In the rare case of index wrap-around, 0 will be returned.
+ * In the rare case of index wrap-around, 0 will be returned. 0 will also
+ * be returned if index == 0 and there is a gap at the index.  We can not
+ * wrap-around if passed index == 0.
   */
  pgoff_t page_cache_next_miss(struct address_space *mapping,
  			     pgoff_t index, unsigned long max_scan)
@@ -1743,12 +1745,13 @@ pgoff_t page_cache_next_miss(struct address_space *mapping,
  	while (max_scan--) {
  		void *entry = xas_next(&xas);
  		if (!entry || xa_is_value(entry))
-			break;
-		if (xas.xa_index == 0)
-			break;
+			return xas.xa_index;
+		if (xas.xa_index == 0 && index != 0)
+			return xas.xa_index;
  	}

-	return xas.xa_index;
+	/* No gaps in range and no wrap-around, return index beyond range */
+	return xas.xa_index + 1;
  }
  EXPORT_SYMBOL(page_cache_next_miss);


This doesn't seem to work as expected:

Here's a test I did

/* Modified so I can pass in an xarray for this test */
static unsigned long page_cache_next_miss(struct xarray *xa, unsigned long index,
					  unsigned long max_scan)
{
	XA_STATE(xas, xa, index);

	while (max_scan--) {
		void *entry = xas_next(&xas);
		if (!entry || xa_is_value(entry))
			return xas.xa_index;
		if (xas.xa_index == 0 && index != 0)
			return xas.xa_index;
	}

	return xas.xa_index + 1;
}

static noinline void check_find_5(void)
{
	struct xarray xa;
	unsigned long max_scan;
	void *ptr = malloc(10);

	xa_init(&xa);
	xa_store_range(&xa, 3, 5, ptr, GFP_KERNEL);

	max_scan = 3;
	printk("page_cache_next_miss(xa, %d, %ld): %ld\n", 4, max_scan,
	       page_cache_next_miss(&xa, 4, max_scan));

}

The above gave me: page_cache_next_miss(xa, 4, 3): 7

But I was expecting a return value of 6.

I investigated a little, and it seems like entry at index 6 if we start
iterating before 6 is 0xe, and xa_is_internal(entry) returns true.

Not yet familiar with the internals of xarrays, not sure what the fix
should be.




[Index of Archives]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Bugtraq]     [Linux OMAP]     [Linux MIPS]     [eCos]     [Asterisk Internet PBX]     [Linux API]

  Powered by Linux