Re: [PATCH] fixup: mm/huge_memory.c: introduce folio_split_unmapped
From: David Hildenbrand (Red Hat)
Date: Thu Nov 20 2025 - 05:43:34 EST
On 11/20/25 11:35, Balbir Singh wrote:
On 11/20/25 20:32, David Hildenbrand (Red Hat) wrote:
On 11/20/25 10:25, Balbir Singh wrote:
On 11/20/25 20:09, David Hildenbrand (Red Hat) wrote:Good point, but the questionable thing is that it looks like nr_shmem_dropped
On 11/20/25 04:07, Balbir Singh wrote:
Code refactoring of __folio_split() via helper
__folio_freeze_and_split_unmapped() caused a regression with clang-20
with CONFIG_SHMEM=n, the compiler was not able to optimize away the
call to shmem_uncharge() due to changes in nr_shmem_dropped.
Fix this by checking for shmem_mapping() prior to calling
shmem_uncharge(), shmem_mapping() returns false when CONFIG_SHMEM=n.
smatch also complained about parameter end being used without
initialization, which is a false positive, but keep the tool happy
by sending in initialized parameters. end is initialized to 0.
Add detailed documentation comments for folio_split_unmapped()
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: Rakie Kim <rakie.kim@xxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Gregory Price <gourry@xxxxxxxxxx>
Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
Cc: Nico Pache <npache@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Lyude Paul <lyude@xxxxxxxxxx>
Cc: Danilo Krummrich <dakr@xxxxxxxxxx>
Cc: David Airlie <airlied@xxxxxxxxx>
Cc: Simona Vetter <simona@xxxxxxxx>
Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
Cc: Mika Penttilä <mpenttil@xxxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Francois Dugast <francois.dugast@xxxxxxxxx>
Signed-off-by: Balbir Singh <balbirs@xxxxxxxxxx>
---
mm/huge_memory.c | 32 ++++++++++++++++++++++----------
1 file changed, 22 insertions(+), 10 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78a31a476ad3..c4267a0f74df 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3751,6 +3751,7 @@ static int __folio_freeze_and_split_unmapped(struct folio *folio, unsigned int n
int ret = 0;
struct deferred_split *ds_queue;
+ VM_WARN_ON_ONCE(!mapping && end != 0);
You could drop the "!= 0"
Ack
VM_WARN_ONE(!mapping && end);
/* Prevent deferred_split_scan() touching ->_refcount */
ds_queue = folio_split_queue_lock(folio);
if (folio_ref_freeze(folio, 1 + extra_pins)) {
@@ -3919,7 +3920,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
int nr_shmem_dropped = 0;
int remap_flags = 0;
int extra_pins, ret;
- pgoff_t end;
+ pgoff_t end = 0;
bool is_hzp;
VM_WARN_ON_ONCE_FOLIO(!folio_test_locked(folio), folio);
@@ -4049,7 +4050,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
local_irq_enable();
- if (nr_shmem_dropped)
+ if (mapping && shmem_mapping(mapping) && nr_shmem_dropped)
shmem_uncharge(mapping->host, nr_shmem_dropped);
That looks questionable. We shouldn't add runtime check to handle buildtime things.
Likely what you want is instead
if (IS_ENABLED(CONFIG_SHMEM) && nr_shmem_dropped)
shmem_uncharge()
shmem_mapping() returns false for CONFIG_SHMEM=n and shmem_mapping() checks that the mapping
is indeed for shmem ops before uncharging. Happy to change it if you like,
your version is more readable
could be set for non-shmem mappings, when it's really just a compiler thing.
What about handling it through a proper stub so we can keep this calling code simple?
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 5b368f9549d67..e38cb01031200 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -136,11 +136,15 @@ static inline bool shmem_hpage_pmd_enabled(void)
#ifdef CONFIG_SHMEM
extern unsigned long shmem_swap_usage(struct vm_area_struct *vma);
+extern void shmem_uncharge(struct inode *inode, long pages);
#else
static inline unsigned long shmem_swap_usage(struct vm_area_struct *vma)
{
return 0;
}
+static inline void shmem_uncharge(struct inode *inode, long pages)
+{
+}
#endif
extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
pgoff_t start, pgoff_t end);
@@ -194,7 +198,6 @@ static inline pgoff_t shmem_fallocend(struct inode *inode, pgoff_t eof)
}
extern bool shmem_charge(struct inode *inode, long pages);
-extern void shmem_uncharge(struct inode *inode, long pages);
#ifdef CONFIG_USERFAULTFD
#ifdef CONFIG_SHMEM
Agreed, I would like to let this patch proceed and then immediately follow up patch
along the lines of CONFIG_SHMEM as separate independent patch (independent of this
series). What do you think?
Let's do it properly right away, no need to hurry that much.
--
Cheers
David