diff mbox series

[v2,1/2] mm/memfd: refactor memfd_tag_pins() and memfd_wait_for_pins()

Message ID 20240226141324.278526-2-david@redhat.com (mailing list archive)
State New
Headers show
Series mm: remove total_mapcount() | expand

Commit Message

David Hildenbrand Feb. 26, 2024, 2:13 p.m. UTC 
Both functions are the remaining users of total_mapcount(). Let's get
rid of the calls by converting the code to folios.

As it turns out, the code is unnecessarily complicated, especially:

1) We can query the number of pagecache references for a folio simply via
   folio_nr_pages(). This will handle other folio sizes in the future
   correctly.

2) The xas_set(xas, page->index + cache_count) call to increment the
   iterator for large folios is not required. Remove it.

Further, simplify the XA_CHECK_SCHED check, counting each entry exactly
once.

Memfd pages can be swapped out when using shmem; leave xa_is_value()
checks in place.

Co-developed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 mm/memfd.c | 47 ++++++++++++++++++-----------------------------
 1 file changed, 18 insertions(+), 29 deletions(-)

Comments

Matthew Wilcox Feb. 26, 2024, 4:07 p.m. UTC | #1 
On Mon, Feb 26, 2024 at 03:13:23PM +0100, David Hildenbrand wrote:
> +	xas_for_each(xas, folio, ULONG_MAX) {
> +		if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
>  			xas_set_mark(xas, MEMFD_TAG_PINNED);

... we decline to tag value entries here ...

> @@ -95,20 +90,15 @@ static int memfd_wait_for_pins(struct address_space *mapping)
>  
>  		xas_set(&xas, 0);
>  		xas_lock_irq(&xas);
> -		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
> +		xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
>  			bool clear = true;
>  
> -			cache_count = 1;
> -			if (!xa_is_value(page) &&
> -			    PageTransHuge(page) && !PageHuge(page))
> -				cache_count = HPAGE_PMD_NR;
> -
> -			if (!xa_is_value(page) && cache_count !=
> -			    page_count(page) - total_mapcount(page)) {
> +			if (!xa_is_value(folio) &&
> +			    memfd_folio_has_extra_refs(folio)) {

... so we don't need to test it here because we'll never see any value
entries.  No?
David Hildenbrand Feb. 26, 2024, 4:56 p.m. UTC | #2 
On 26.02.24 17:07, Matthew Wilcox wrote:
> On Mon, Feb 26, 2024 at 03:13:23PM +0100, David Hildenbrand wrote:
>> +	xas_for_each(xas, folio, ULONG_MAX) {
>> +		if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
>>   			xas_set_mark(xas, MEMFD_TAG_PINNED);
> 
> ... we decline to tag value entries here ...
> 
>> @@ -95,20 +90,15 @@ static int memfd_wait_for_pins(struct address_space *mapping)
>>   
>>   		xas_set(&xas, 0);
>>   		xas_lock_irq(&xas);
>> -		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
>> +		xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
>>   			bool clear = true;
>>   
>> -			cache_count = 1;
>> -			if (!xa_is_value(page) &&
>> -			    PageTransHuge(page) && !PageHuge(page))
>> -				cache_count = HPAGE_PMD_NR;
>> -
>> -			if (!xa_is_value(page) && cache_count !=
>> -			    page_count(page) - total_mapcount(page)) {
>> +			if (!xa_is_value(folio) &&
>> +			    memfd_folio_has_extra_refs(folio)) {
> 
> ... so we don't need to test it here because we'll never see any value
> entries.  No?

I was not able to convince myself that swapout code would clear the mark 
when replacing the entry.

shmem_writepage()->shmem_delete_from_page_cache()->shmem_replace_entry()

will perform a xas_store() with swp_to_radix_entry(swap) under 
xa_lock_irq().

Reading the doc, and staring at the code for a bit too long, I think 
xas_store() would only clear tags when deleting an entry (passing NULL).

But maybe xas_store() will always clear tags?

In memfd code, I think we could see swapout between memfd_tag_pins() and 
the check for tags, where we drop the xa_lock. Unless some other lock 
(inode lock?) protects us.

Thanks!
Matthew Wilcox Feb. 27, 2024, 3:27 p.m. UTC | #3 
On Mon, Feb 26, 2024 at 05:56:09PM +0100, David Hildenbrand wrote:
> > > @@ -95,20 +90,15 @@ static int memfd_wait_for_pins(struct address_space *mapping)
> > >   		xas_set(&xas, 0);
> > >   		xas_lock_irq(&xas);
> > > -		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
> > > +		xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
> > >   			bool clear = true;
> > > -			cache_count = 1;
> > > -			if (!xa_is_value(page) &&
> > > -			    PageTransHuge(page) && !PageHuge(page))
> > > -				cache_count = HPAGE_PMD_NR;
> > > -
> > > -			if (!xa_is_value(page) && cache_count !=
> > > -			    page_count(page) - total_mapcount(page)) {
> > > +			if (!xa_is_value(folio) &&
> > > +			    memfd_folio_has_extra_refs(folio)) {
> > 
> > ... so we don't need to test it here because we'll never see any value
> > entries.  No?
> 
> I was not able to convince myself that swapout code would clear the mark
> when replacing the entry.
> 
> shmem_writepage()->shmem_delete_from_page_cache()->shmem_replace_entry()
> 
> will perform a xas_store() with swp_to_radix_entry(swap) under
> xa_lock_irq().
> 
> Reading the doc, and staring at the code for a bit too long, I think
> xas_store() would only clear tags when deleting an entry (passing NULL).
> 
> But maybe xas_store() will always clear tags?

No, xas_store() will leave the tag alone ... this is the right thing to
do for the pagecache because we always clear the tags before removing
a folio from the cache.

> In memfd code, I think we could see swapout between memfd_tag_pins() and the
> check for tags, where we drop the xa_lock. Unless some other lock (inode
> lock?) protects us.

... and if it does happen, we see the value entry tagged and clear the
tag on it.  OK.

Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
diff mbox series

Patch

diff --git a/mm/memfd.c b/mm/memfd.c
index d3a1ba4208c90..7d8d3ab3fa378 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -29,29 +29,25 @@ 
 #define MEMFD_TAG_PINNED        PAGECACHE_TAG_TOWRITE
 #define LAST_SCAN               4       /* about 150ms max */
 
+static bool memfd_folio_has_extra_refs(struct folio *folio)
+{
+	return folio_ref_count(folio) - folio_mapcount(folio) !=
+	       folio_nr_pages(folio);
+}
+
 static void memfd_tag_pins(struct xa_state *xas)
 {
-	struct page *page;
+	struct folio *folio;
 	int latency = 0;
-	int cache_count;
 
 	lru_add_drain();
 
 	xas_lock_irq(xas);
-	xas_for_each(xas, page, ULONG_MAX) {
-		cache_count = 1;
-		if (!xa_is_value(page) &&
-		    PageTransHuge(page) && !PageHuge(page))
-			cache_count = HPAGE_PMD_NR;
-
-		if (!xa_is_value(page) &&
-		    page_count(page) - total_mapcount(page) != cache_count)
+	xas_for_each(xas, folio, ULONG_MAX) {
+		if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
 			xas_set_mark(xas, MEMFD_TAG_PINNED);
-		if (cache_count != 1)
-			xas_set(xas, page->index + cache_count);
 
-		latency += cache_count;
-		if (latency < XA_CHECK_SCHED)
+		if (++latency < XA_CHECK_SCHED)
 			continue;
 		latency = 0;
 
@@ -66,16 +62,16 @@  static void memfd_tag_pins(struct xa_state *xas)
 /*
  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
  * via get_user_pages(), drivers might have some pending I/O without any active
- * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
+ * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios
  * and see whether it has an elevated ref-count. If so, we tag them and wait for
  * them to be dropped.
  * The caller must guarantee that no new user will acquire writable references
- * to those pages to avoid races.
+ * to those folios to avoid races.
  */
 static int memfd_wait_for_pins(struct address_space *mapping)
 {
 	XA_STATE(xas, &mapping->i_pages, 0);
-	struct page *page;
+	struct folio *folio;
 	int error, scan;
 
 	memfd_tag_pins(&xas);
@@ -83,7 +79,6 @@  static int memfd_wait_for_pins(struct address_space *mapping)
 	error = 0;
 	for (scan = 0; scan <= LAST_SCAN; scan++) {
 		int latency = 0;
-		int cache_count;
 
 		if (!xas_marked(&xas, MEMFD_TAG_PINNED))
 			break;
@@ -95,20 +90,15 @@  static int memfd_wait_for_pins(struct address_space *mapping)
 
 		xas_set(&xas, 0);
 		xas_lock_irq(&xas);
-		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
+		xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
 			bool clear = true;
 
-			cache_count = 1;
-			if (!xa_is_value(page) &&
-			    PageTransHuge(page) && !PageHuge(page))
-				cache_count = HPAGE_PMD_NR;
-
-			if (!xa_is_value(page) && cache_count !=
-			    page_count(page) - total_mapcount(page)) {
+			if (!xa_is_value(folio) &&
+			    memfd_folio_has_extra_refs(folio)) {
 				/*
 				 * On the last scan, we clean up all those tags
 				 * we inserted; but make a note that we still
-				 * found pages pinned.
+				 * found folios pinned.
 				 */
 				if (scan == LAST_SCAN)
 					error = -EBUSY;
@@ -118,8 +108,7 @@  static int memfd_wait_for_pins(struct address_space *mapping)
 			if (clear)
 				xas_clear_mark(&xas, MEMFD_TAG_PINNED);
 
-			latency += cache_count;
-			if (latency < XA_CHECK_SCHED)
+			if (++latency < XA_CHECK_SCHED)
 				continue;
 			latency = 0;