Re: [PATCH] mm: shmem: fix incorrect vm_flags usage when checking allowable orders
From: Baolin Wang
Date: Mon Aug 17 2026 - 05:35:18 EST
On 8/17/26 3:40 PM, Lorenzo Stoakes (ARM) wrote:
On Mon, Aug 17, 2026 at 03:16:43PM +0800, Baolin Wang wrote:
Lance reported that when MADV_HUGEPAGE is set on a tmpfs file mounted with
huge=advise option, khugepaged fails the allowable order check and does not
scan the tmpfs file for collapse.
Ugh.
But really I think this isn't quite accurate - what you mean to say I think is
that when _nothing else_ causes the mm to be considered for khugepaged collapse,
an MADV_HUGEPAGE-advised tmpfs VMA alone does not cause scanning to commence.
Yes.
After commit 6beeab870e70 ("mm: shmem: move shmem_huge_global_enabled() into
shmem_allowable_huge_orders()"), the shmem/tmpfs allowable order check reads
vma->vm_flags directly. However, when MADV_HUGEPAGE is handled,
khugepaged_enter_vma() is called before the VMA's vm_flags have been updated,
so the check uses stale flags and incorrectly rejects the VMA for collapse.
As a result, khugepaged does not collapse the tmpfs file into PMD order in time.
Could we at least refer to the non-deprecated field in the commit message?
i.e. vma->flags.
Probably worth mentioning VMA_HUGEPAGE_BIT also.
Sure.
Fix this by passing vm_flags as a parameter to shmem_allowable_huge_orders()
instead of reading it from the vm_area_struct.
Ugh this is so disgusting.
I understand this is a fix for a bug to be backported but couldn't we just
achieve the same without having to add a deprecated field to be passed around?
As you say the khugepaged_enter_vma() isn't really so helpful in
hugepage_madvise().
But you could add this to the bottom of madvise_update_vma():
if (vma_flags_test(&new_vma_flags, VMA_HUGEPAGE_BIT))
khugepaged_enter_vma(vma, new_flags);
I don't think this is really egregious on this code path and could go some way
towards us eliminating the silly thing of passing around flags-to-be-set.
This is not the point (maybe I didn't describe it clearly). The point is that the allowable orders check for tmpfs in shmem_huge_global_enabled() (called by shmem_allowable_huge_orders()) uses a stale vma flag.
static unsigned int shmem_huge_global_enabled(struct inode *inode, pgoff_t index,
loff_t write_end, bool shmem_huge_force,
struct vm_area_struct *vma,
vm_flags_t vm_flags)
{
......
switch (SHMEM_SB(inode->i_sb)->huge) {
......
case SHMEM_HUGE_ADVISE:
if (vm_flags & VM_HUGEPAGE)
return THP_ORDERS_ALL_FILE_DEFAULT;
fallthrough;
default:
return 0;
}
}
So we should pass the new vma flags for shmem_allowable_huge_orders() to check the allowable orders for tmpfs. Changing madvise_update_vma() doesn't help with the allowable orders check for tmpfs.
And yeah you'd endure a little backport pain on switching that new_vma_flags
vma_flags_t test to a new_flags vm_flags_t test but it's not so bad :)
I will be doing a series soon that improves the allowable orders vomit-a-thon
somewhat in tip :)
Great. Look forward to seeing these cleanups.
Reported-by: Lance Yang <lance.yang@xxxxxxxxx>
Closes: https://lore.kernel.org/all/20260815181632.21453-1-lance.yang@xxxxxxxxx/
Fixes: 6beeab870e70 ("mm: shmem: move shmem_huge_global_enabled() into shmem_allowable_huge_orders()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
include/linux/shmem_fs.h | 8 ++++----
mm/huge_memory.c | 2 +-
mm/shmem.c | 9 +++++----
3 files changed, 10 insertions(+), 9 deletions(-)
diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
index 5663dff53186..321017e0fd63 100644
--- a/include/linux/shmem_fs.h
+++ b/include/linux/shmem_fs.h
@@ -127,13 +127,13 @@ int shmem_unuse(unsigned int type);
#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_SHMEM)
unsigned long shmem_allowable_huge_orders(struct inode *inode,
- struct vm_area_struct *vma, pgoff_t index,
- loff_t write_end, bool shmem_huge_force);
+ struct vm_area_struct *vma, vm_flags_t vm_flags,
+ pgoff_t index, loff_t write_end, bool shmem_huge_force);
bool shmem_hpage_pmd_enabled(void);
#else
static inline unsigned long shmem_allowable_huge_orders(struct inode *inode,
- struct vm_area_struct *vma, pgoff_t index,
- loff_t write_end, bool shmem_huge_force)
+ struct vm_area_struct *vma, vm_flags_t vm_flags, pgoff_t index,
+ loff_t write_end, bool shmem_huge_force)
{
return 0;
}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index ced400f72d43..70f57d700739 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -187,7 +187,7 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
*/
if (!in_pf && shmem_file(vma->vm_file))
return orders & shmem_allowable_huge_orders(file_inode(vma->vm_file),
- vma, vma_start_pgoff(vma), 0,
+ vma, vm_flags, vma_start_pgoff(vma), 0,
forced_collapse);
if (!vma_is_anonymous(vma)) {
diff --git a/mm/shmem.c b/mm/shmem.c
index 599665a3d6e7..6f1ad5456aca 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1834,12 +1834,11 @@ bool shmem_hpage_pmd_enabled(void)
}
unsigned long shmem_allowable_huge_orders(struct inode *inode,
- struct vm_area_struct *vma, pgoff_t index,
- loff_t write_end, bool shmem_huge_force)
+ struct vm_area_struct *vma, vm_flags_t vm_flags,
+ pgoff_t index, loff_t write_end, bool shmem_huge_force)
{
unsigned long mask = READ_ONCE(huge_shmem_orders_always);
unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size);
- vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
unsigned int global_orders;
if (thp_disabled_by_hw() || (vma && vma_thp_disabled(vma, vm_flags, shmem_huge_force)))
@@ -2430,6 +2429,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
gfp_t gfp, struct vm_fault *vmf, vm_fault_t *fault_type)
{
struct vm_area_struct *vma = vmf ? vmf->vma : NULL;
+ vm_flags_t vm_flags = vma ? vma->vm_flags : 0;
struct mm_struct *fault_mm;
struct folio *folio;
int error;
@@ -2507,7 +2507,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
}
/* Find hugepage orders that are allowed for anonymous shmem and tmpfs. */
- orders = shmem_allowable_huge_orders(inode, vma, index, write_end, false);
+ orders = shmem_allowable_huge_orders(inode, vma, vm_flags, index,
+ write_end, false);
if (orders > 0) {
gfp_t huge_gfp;
--
2.47.3
--
Cheers, Lorenzo