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

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

[Mike Kravetz]

On 06/05/23 17:26, Ackerley Tng wrote:
> Mike Kravetz <mike.kravetz@oracle.com> writes:
> 
> 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.

I am NOT an expert with xarray.  However, the documentation says:

"Calling xa_store_range() stores the same entry in a range
 of indices.  If you do this, some of the other operations will behave
 in a slightly odd way.  For example, marking the entry at one index
 may result in the entry being marked at some, but not all of the other
 indices.  Storing into one index may result in the entry retrieved by
 some, but not all of the other indices changing."

This may be why your test is not functioning as expected?  I modified
your check_find_5() routine as follows (within lib/test_xarray.c):

static noinline void check_find_5(struct xarray *xa, bool mult)
{
	unsigned long max_scan;
	void *p = &max_scan;

	XA_BUG_ON(xa, !xa_empty(xa));

	if (mult) {
		xa_store(xa, 3, p, GFP_KERNEL);
		xa_store(xa, 4, p, GFP_KERNEL);
		xa_store(xa, 5, p, GFP_KERNEL);
	} else {
		xa_store_range(xa, 3, 5, p, GFP_KERNEL);
	}

	max_scan = 3;
	if (mult)
		printk("---> multiple stores\n");
	else
		printk("---> range store\n");

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

	if (mult) {
		xa_store(xa, 3, NULL, GFP_KERNEL);
		xa_store(xa, 4, NULL, GFP_KERNEL);
		xa_store(xa, 5, NULL, GFP_KERNEL);
	} else {
		xa_store_range(xa, 3, 5, NULL, GFP_KERNEL);
	}

	xa_destroy(xa);
}

This results in:
[  149.998676] ---> multiple stores
[  149.999391] page_cache_next_miss(xa, 4, 3): 6
[  150.003342] ---> range store
[  150.007002] page_cache_next_miss(xa, 4, 3): 7

I am fairly confident the page cache code will make individual xa_store
calls as opposed to xa_store_range.
-- 
Mike Kravetz

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

[Ackerley Tng]

Mike Kravetz <mike.kravetz@oracle.com> writes:

> On 06/05/23 17:26, Ackerley Tng wrote:
>> Mike Kravetz <mike.kravetz@oracle.com> writes:

>> This doesn't seem to work as expected:

>> Here's a test I did

>>   ...

>> 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.

> I am NOT an expert with xarray.  However, the documentation says:

> "Calling xa_store_range() stores the same entry in a range
>   of indices.  If you do this, some of the other operations will behave
>   in a slightly odd way.  For example, marking the entry at one index
>   may result in the entry being marked at some, but not all of the other
>   indices.  Storing into one index may result in the entry retrieved by
>   some, but not all of the other indices changing."

> This may be why your test is not functioning as expected?  I modified
> your check_find_5() routine as follows (within lib/test_xarray.c):

> static noinline void check_find_5(struct xarray *xa, bool mult)
> {
> 	unsigned long max_scan;
> 	void *p = &max_scan;

> 	XA_BUG_ON(xa, !xa_empty(xa));

> 	if (mult) {
> 		xa_store(xa, 3, p, GFP_KERNEL);
> 		xa_store(xa, 4, p, GFP_KERNEL);
> 		xa_store(xa, 5, p, GFP_KERNEL);
> 	} else {
> 		xa_store_range(xa, 3, 5, p, GFP_KERNEL);
> 	}

> 	max_scan = 3;
> 	if (mult)
> 		printk("---> multiple stores\n");
> 	else
> 		printk("---> range store\n");

> 	printk("page_cache_next_miss(xa, %d, %ld): %ld\n", 4, max_scan,
> 		__page_cache_next_miss(xa, 4, max_scan));

> 	if (mult) {
> 		xa_store(xa, 3, NULL, GFP_KERNEL);
> 		xa_store(xa, 4, NULL, GFP_KERNEL);
> 		xa_store(xa, 5, NULL, GFP_KERNEL);
> 	} else {
> 		xa_store_range(xa, 3, 5, NULL, GFP_KERNEL);
> 	}

> 	xa_destroy(xa);
> }

> This results in:
> [  149.998676] ---> multiple stores
> [  149.999391] page_cache_next_miss(xa, 4, 3): 6
> [  150.003342] ---> range store
> [  150.007002] page_cache_next_miss(xa, 4, 3): 7

> I am fairly confident the page cache code will make individual xa_store
> calls as opposed to xa_store_range.

I tried this out with xa_store and a non-NULL pointer, and it works as
expected. Thanks!

I also checked that filemap/page_cache doesn't use xa_store_range(). It
only uses xas_store().

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>

[PATCH 0/1] fix page_cache_next/prev_miss off by one error

[Mike Kravetz]

A cover letter is not necessary for this single patch as all information
is present in the commit message.  However, I am not 100% comfortable in
this change and would REALLY like to get comments from Matthew.

When reporting this issue, Ackerley Tng suggested a solution by creating
a new filemap_has_folio() function[1].  I believe that would be an
acceptable way to proceed although we would also need to change the
other use of page_cache_next_miss in hugetlb.c.

When looking more closely, it looks like page_cache_next/prev_miss do
not work exactly as described.  The result is the following patch.

IIUC, prior to hugetlb use of page_cache_next/prev_miss, it was only
used by readahead code.  My patch does change the return value and has
potential to impact the readahead users.  That is why I am not 100%
comfortable with this.

In any case, this is broken in v6.3 so we need a fix.

[1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@google.com/
Mike Kravetz (1):
  page cache: fix page_cache_next/prev_miss off by one

 mm/filemap.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

-- 
2.40.0

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

[Mike Kravetz]

Ackerley Tng reported an issue with hugetlbfs fallocate here[1].  The
issue showed up after the conversion of hugetlb page cache lookup code
to use page_cache_next_miss.  Code in hugetlb fallocate, userfaultfd
and GUP is now using page_cache_next_miss to determine if a page is
present the page cache.  The following statement is used.

	present = page_cache_next_miss(mapping, index, 1) != index;

There are two issues with page_cache_next_miss when used in this way.
1) If the passed value for index is equal to the 'wrap-around' value,
   the same index will always be returned.  This wrap-around value is 0,
   so 0 will be returned even if page is present at index 0.
2) If there is no gap in the range passed, the last index in the range
   will be returned.  When passed a range of 1 as above, the passed
   index value will be returned even if the page is present.
The end result is the statement above will NEVER indicate a page is
present in the cache, even if it is.

As noted by Ackerley in [1], users can see this by hugetlb fallocate
incorrectly returning EEXIST if pages are already present in the file.
In addition, hugetlb pages will not be included in core dumps if they
need to be brought in via GUP.  userfaultfd UFFDIO_COPY also uses this
code and will not notice pages already present in the cache.  It may try
to allocate a new page and potentially return ENOMEM as opposed to
EEXIST.

Both page_cache_next_miss and page_cache_prev_miss have similar issues.
Fix by:
- Check for index equal to 'wrap-around' value and do not exit early.
- If no gap is found in range, return index outside range.
- Update function description to say 'wrap-around' value could be
  returned if passed as index.

Fixes: 0d3f92966629 ("page cache: Convert hole search to XArray")
Cc: <stable@vger.kernel.org>
Reported-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

[1] https://lore.kernel.org/linux-mm/cover.1683069252.git.ackerleytng@google.com/
---
 mm/filemap.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index a34abfe8c654..60875d349a7b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1760,7 +1760,9 @@ 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)
@@ -1770,12 +1772,13 @@ 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);
 
@@ -1796,7 +1799,9 @@ EXPORT_SYMBOL(page_cache_next_miss);
  *
  * Return: The index of the gap if found, otherwise an index outside the
  * range specified (in which case 'index - return >= max_scan' will be true).
- * In the rare case of wrap-around, ULONG_MAX will be returned.
+ * In the rare case of wrap-around, ULONG_MAX will be returned.  ULONG_MAX
+ * will also be returned if index == ULONG_MAX and there is a gap at the
+ * index.  We can not wrap-around if passed index == ULONG_MAX.
  */
 pgoff_t page_cache_prev_miss(struct address_space *mapping,
 			     pgoff_t index, unsigned long max_scan)
@@ -1806,12 +1811,13 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
 	while (max_scan--) {
 		void *entry = xas_prev(&xas);
 		if (!entry || xa_is_value(entry))
-			break;
-		if (xas.xa_index == ULONG_MAX)
-			break;
+			return xas.xa_index;
+		if (xas.xa_index == ULONG_MAX && index != ULONG_MAX)
+			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_prev_miss);
 
-- 
2.40.0