[merged mm-hotfixes-stable] mm-decline-to-manipulate-the-refcount-on-a-slab-page.patch removed from -mm tree

[Andrew Morton <akpm@linux-foundation.org>]

The quilt patch titled
     Subject: mm: decline to manipulate the refcount on a slab page
has been removed from the -mm tree.  Its filename was
     mm-decline-to-manipulate-the-refcount-on-a-slab-page.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Subject: mm: decline to manipulate the refcount on a slab page
Date: Mon, 10 Mar 2025 14:35:24 +0000

Slab pages now have a refcount of 0, so nobody should be trying to
manipulate the refcount on them.  Doing so has little effect; the object
could be freed and reallocated to a different purpose, although the slab
itself would not be until the refcount was put making it behave rather
like TYPESAFE_BY_RCU.

Unfortunately, __iov_iter_get_pages_alloc() does take a refcount.  Fix
that to not change the refcount, and make put_page() silently not change
the refcount.  get_page() warns so that we can fix any other callers that
need to be changed.

Long-term, networking needs to stop taking a refcount on the pages that it
uses and rely on the caller to hold whatever references are necessary to
make the memory stable.  In the medium term, more page types are going to
hav a zero refcount, so we'll want to move get_page() and put_page() out
of line.

Link: https://lkml.kernel.org/r/20250310143544.1216127-1-willy@infradead.org
Fixes: 9aec2fb0fd5e (slab: allocate frozen pages)
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reported-by: Hannes Reinecke <hare@suse.de>
Closes: https://lore.kernel.org/all/08c29e4b-2f71-4b6d-8046-27e407214d8c@suse.com/
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/mm.h |    8 +++++++-
 lib/iov_iter.c     |    8 ++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

--- a/include/linux/mm.h~mm-decline-to-manipulate-the-refcount-on-a-slab-page
+++ a/include/linux/mm.h
@@ -1458,7 +1458,10 @@ static inline void folio_get(struct foli
 
 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)
@@ -1552,6 +1555,9 @@ static inline void put_page(struct page
 {
 	struct folio *folio = page_folio(page);
 
+	if (folio_test_slab(folio))
+		return;
+
 	/*
 	 * For some devmap managed pages we need to catch refcount transition
 	 * from 2 to 1:
--- a/lib/iov_iter.c~mm-decline-to-manipulate-the-refcount-on-a-slab-page
+++ a/lib/iov_iter.c
@@ -1190,8 +1190,12 @@ static ssize_t __iov_iter_get_pages_allo
 		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;
_

Patches currently in -mm which might be from willy@infradead.org are

dax-remove-access-to-page-index.patch
dax-use-folios-more-widely-within-dax.patch
fs-convert-block_commit_write-to-take-a-folio.patch
fs-remove-page_file_mapping.patch
fs-remove-folio_file_mapping.patch
mm-assert-the-folio-is-locked-in-folio_start_writeback.patch
hugetlb-convert-hugetlb_vma_maps_page-to-hugetlb_vma_maps_pfn.patch
hugetlb-convert-adjust_range_hwpoison-to-take-a-folio.patch
mm-convert-lru_add_page_tail-to-lru_add_split_folio.patch
mm-separate-folio_split_memcg_refs-from-split_page_memcg.patch
mm-simplify-split_page_memcg.patch
mm-remove-references-to-folio-in-split_page_memcg.patch
mm-simplify-folio_memcg_charged.patch
mm-remove-references-to-folio-in-__memcg_kmem_uncharge_page.patch
ocfs2-use-memcpy_to_folio-in-ocfs2_symlink_get_block.patch
ocfs2-remove-reference-to-bh-b_page.patch