On 3/4/25 20:39, Hannes Reinecke wrote:

On 3/4/25 19:05, Matthew Wilcox wrote:

quoted

On Tue, Mar 04, 2025 at 04:53:09PM +0000, Matthew Wilcox wrote:

quoted

Right, that's what happened in the block layer.  We mark the bio with
BIO_PAGE_PINNED if the pincount needs to be dropped.  As a transitional
period, we had BIO_PAGE_REFFED which indicated that the page refcount
needed to be dropped.  Perhaps there's something similar that network
could be doing.

Until that time ... how does this look as a quick hack to avoid
reverting the slab change?

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d6fed25243c3..ca08a923ac6d 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -1520,7 +1520,10 @@ static inline void folio_get(struct folio *folio)
  
  static inline void get_page(struct page *page)
  {
-	folio_get(page_folio(page));
+	struct folio *folio = page_folio(page);
+	if (WARN_ON_ONCE(folio_test_slab(folio)))
+		return;
+	folio_get(folio);
  }
  
  static inline __must_check bool try_get_page(struct page *page)

@@ -1614,6 +1617,8 @@ static inline void put_page(struct page *page)
  {
  	struct folio *folio = page_folio(page);
  
+	if (folio_test_slab(folio))
+		return;
  	folio_put(folio);
  }
  

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 65f550cb5081..8c7fdb7d8c8f 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c

@@ -1190,8 +1190,12 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
  		if (!n)
  			return -ENOMEM;
  		p = *pages;
-		for (int k = 0; k < n; k++)
-			get_page(p[k] = page + k);
+		for (int k = 0; k < n; k++) {
+			struct folio *folio = page_folio(page);
+			p[k] = page + k;
+			if (!folio_test_slab(folio))
+				folio_get(folio);
+		}
  		maxsize = min_t(size_t, maxsize, n * PAGE_SIZE - *start);
  		i->count -= maxsize;
  		i->iov_offset += maxsize;

Good news and bad news ...
Good news: TLS works again!
Bad news: no errors.

Wait, did you add a WARN_ON_ONCE() to the put_page() as I suggested? If yes
and there was no error, it would have to be leaking the page. Or the path
uses folio_put() and we'd need to put the warning there.

Question to the wise: this is not the only place in iov_iter.c where we 
do a 'get_page()'. Do we leave them and wait for others to report 
regressions, knowing fully well that the current code _has_ issues?
Or shouldn't we rather clean them up?

I guess the real fix would be to fiddle with the 'bio_add_page()' logic;
we are always adding a 'page' reference to the bio, completely ignoring
whether this page is a slab page or a normal one.

Discussion at LSF, maybe?

Cheers,

Hannes