FAILED: patch "[PATCH] mm: Fix a hmm_range_fault() livelock / starvation problem" failed to apply to 6.18-stable tree
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maintainerThe patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.
To reproduce the conflict and resubmit, you may use the following commands:
git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x b570f37a2ce480be26c665345c5514686a8a0274
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026031727-elective-spilt-6870@gregkh' --subject-prefix 'PATCH 6.18.y' HEAD^..
Possible dependencies:
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From b570f37a2ce480be26c665345c5514686a8a0274 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Thomas=20Hellstr=C3=B6m?= <[email protected]>
Date: Tue, 10 Feb 2026 12:56:53 +0100
Subject: [PATCH] mm: Fix a hmm_range_fault() livelock / starvation problem
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If hmm_range_fault() fails a folio_trylock() in do_swap_page, trying to acquire the lock of a device-private folio for migration, to ram, the function will spin until it succeeds grabbing the lock.
However, if the process holding the lock is depending on a work item to be completed, which is scheduled on the same CPU as the spinning hmm_range_fault(), that work item might be starved and we end up in a livelock / starvation situation which is never resolved.
This can happen, for example if the process holding the device-private folio lock is stuck in
migrate_device_unmap()->lru_add_drain_all()
sinc lru_add_drain_all() requires a short work-item to be run on all online cpus to complete.
A prerequisite for this to happen is:
a) Both zone device and system memory folios are considered in migrate_device_unmap(), so that there is a reason to call lru_add_drain_all() for a system memory folio while a folio lock is held on a zone device folio.
b) The zone device folio has an initial mapcount > 1 which causes at least one migration PTE entry insertion to be deferred to try_to_migrate(), which can happen after the call to lru_add_drain_all().
c) No or voluntary only preemption.
This all seems pretty unlikely to happen, but indeed is hit by the "xe_exec_system_allocator" igt test.
Resolve this by waiting for the folio to be unlocked if the folio_trylock() fails in do_swap_page().
Rename migration_entry_wait_on_locked() to softleaf_entry_wait_unlock() and update its documentation to indicate the new use-case.
Future code improvements might consider moving
the lru_add_drain_all() call in migrate_device_unmap() to be called *after* all pages have migration entries inserted. That would eliminate also b) above.
v2:
- Instead of a cond_resched() in hmm_range_fault(), eliminate the problem by waiting for the folio to be unlocked in do_swap_page() (Alistair Popple, Andrew Morton)
v3:
- Add a stub migration_entry_wait_on_locked() for the !CONFIG_MIGRATION case. (Kernel Test Robot)
v4:
- Rename migrate_entry_wait_on_locked() to softleaf_entry_wait_on_locked() and update docs (Alistair Popple)
v5:
- Add a WARN_ON_ONCE() for the !CONFIG_MIGRATION
version of softleaf_entry_wait_on_locked().
- Modify wording around function names in the commit message (Andrew Morton)
include/linux/migrate.h +9 -1
@@ -65,7 +65,7 @@ bool isolate_folio_to_list(struct folio *folio, struct list_head *list);int migrate_huge_page_move_mapping (struct address_space *mapping,struct folio *dst,struct folio *src); -void migration_entry_wait_on_locked(softleaf_t entry, spinlock_t *ptl) +void softleaf_entry_wait_on_locked(softleaf_t entry, spinlock_t *ptl) __releases(ptl);void folio_migrate_flags (struct folio *newfolio,struct folio *folio); int folio_migrate_mapping(struct address_space *mapping,
@@ -97,6 +97,14 @@ static inline int set_movable_ops(const struct movable_operations *ops, enum pag return -ENOSYS; } +static inline void softleaf_entry_wait_on_locked (softleaf_t entry,spinlock_t *ptl) +__releases (ptl) +{ +WARN_ON_ONCE (1 ); + +spin_unlock (ptl); +} +#endif /* CONFIG_MIGRATION */ #ifdef CONFIG_NUMA_BALANCING
mm/filemap.c +10 -5
@@ -1379,14 +1379,16 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,#ifdef CONFIG_MIGRATION /** -* migration_entry_wait_on_locked - Wait for a migration entry to be removed -* @entry: migration swap entry. +* softleaf_entry_wait_on_locked - Wait for a migration entry or +* device_private entry to be removed. +* @entry: migration or device_private swap entry. * @ptl: already locked ptl. This function will drop the lock. * -* Wait for a migration entry referencing the given page to be removed. This is +* Wait for a migration entry referencing the given page, or device_private +* entry referencing a dvice_private page to be unlocked. This is * equivalent to folio_put_wait_locked(folio, TASK_UNINTERRUPTIBLE) except * this can be called without taking a reference on the page. Instead this -* should be called while holding the ptl for the migration entry referencing +* should be called while holding the ptl for @entry referencing * the page. * * Returns after unlocking the ptl.
@@ -1394,7 +1396,7 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,* This follows the same logic as folio_wait_bit_common() so see the comments * there. */ -void migration_entry_wait_on_locked (softleaf_t entry,spinlock_t *ptl) +void softleaf_entry_wait_on_locked (softleaf_t entry,spinlock_t *ptl)__releases (ptl) {struct wait_page_queue wait_page;
@@ -1428,6 +1430,9 @@ void migration_entry_wait_on_locked(softleaf_t entry, spinlock_t *ptl)* If a migration entry exists for the page the migration path must hold * a valid reference to the page, and it must take the ptl to remove the * migration entry. So the page is valid until the ptl is dropped. +* Similarly any path attempting to drop the last reference to a +* device-private page needs to grab the ptl to remove the device-private +* entry. */ spin_unlock (ptl);
mm/memory.c +2 -1
@@ -4763,7 +4763,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)unlock_page (vmf->page);put_page (vmf->page); }else { -pte_unmap_unlock (vmf->pte, vmf->ptl); +pte_unmap (vmf->pte); +softleaf_entry_wait_on_locked (entry, vmf->ptl); } }else if (softleaf_is_hwpoison (entry)) { ret =VM_FAULT_HWPOISON ;
mm/migrate.c +4 -4
@@ -500,7 +500,7 @@ void migration_entry_wait(struct mm_struct *mm, pmd_t *pmd,if (!softleaf_is_migration (entry)) goto out; -migration_entry_wait_on_locked (entry, ptl); +softleaf_entry_wait_on_locked (entry, ptl);return ; out:spin_unlock (ptl);
@@ -532,10 +532,10 @@ void migration_entry_wait_huge(struct vm_area_struct *vma, unsigned long addr, p* If migration entry existed, safe to release vma lock * here because the pgtable page won't be freed without the * pgtable lock released. See comment right above pgtable -* lock release in migration_entry_wait_on_locked(). +* lock release in softleaf_entry_wait_on_locked(). */ hugetlb_vma_unlock_read (vma); -migration_entry_wait_on_locked (entry, ptl); +softleaf_entry_wait_on_locked (entry, ptl);return ; }
@@ -553,7 +553,7 @@ void pmd_migration_entry_wait(struct mm_struct *mm, pmd_t *pmd) ptl =pmd_lock (mm, pmd);if (!pmd_is_migration_entry (*pmd)) goto unlock; -migration_entry_wait_on_locked (softleaf_from_pmd (*pmd), ptl); +softleaf_entry_wait_on_locked (softleaf_from_pmd (*pmd), ptl);return ; unlock:spin_unlock (ptl);
mm/migrate_device.c +1 -1
@@ -176,7 +176,7 @@ static int migrate_vma_collect_huge_pmd(pmd_t *pmdp, unsigned long start, }if (softleaf_is_migration (entry)) { -migration_entry_wait_on_locked (entry, ptl); +softleaf_entry_wait_on_locked (entry, ptl);spin_unlock (ptl);return -EAGAIN ; }
PATCH 1/1 mm: Fix a hmm_range_fault() livelock / starvation problem
If hmm_range_fault() fails a folio_trylock() in do_swap_page, trying to acquire the lock of a device-private folio for migration, to ram, the function will spin until it succeeds grabbing the lock.
However, if the process holding the lock is depending on a work item to be completed, which is scheduled on the same CPU as the spinning hmm_range_fault(), that work item might be starved and we end up in a livelock / starvation situation which is never resolved.
This can happen, for example if the process holding the device-private folio lock is stuck in
migrate_device_unmap()->lru_add_drain_all()
sinc lru_add_drain_all() requires a short work-item to be run on all online cpus to complete.
A prerequisite for this to happen is:
a) Both zone device and system memory folios are considered in migrate_device_unmap(), so that there is a reason to call lru_add_drain_all() for a system memory folio while a folio lock is held on a zone device folio.
b) The zone device folio has an initial mapcount > 1 which causes at least one migration PTE entry insertion to be deferred to try_to_migrate(), which can happen after the call to lru_add_drain_all().
c) No or voluntary only preemption.
This all seems pretty unlikely to happen, but indeed is hit by the "xe_exec_system_allocator" igt test.
Resolve this by waiting for the folio to be unlocked if the folio_trylock() fails in do_swap_page().
Rename migration_entry_wait_on_locked() to softleaf_entry_wait_unlock() and update its documentation to indicate the new use-case.
Future code improvements might consider moving
the lru_add_drain_all() call in migrate_device_unmap() to be called *after* all pages have migration entries inserted. That would eliminate also b) above.
v2:
- Instead of a cond_resched() in hmm_range_fault(), eliminate the problem by waiting for the folio to be unlocked in do_swap_page() (Alistair Popple, Andrew Morton)
v3:
- Add a stub migration_entry_wait_on_locked() for the !CONFIG_MIGRATION case. (Kernel Test Robot)
v4:
- Rename migrate_entry_wait_on_locked() to softleaf_entry_wait_on_locked() and update docs (Alistair Popple)
v5:
- Add a WARN_ON_ONCE() for the !CONFIG_MIGRATION
version of softleaf_entry_wait_on_locked().
- Modify wording around function names in the commit message (Andrew Morton)
--- include/linux/migrate.h | 8 ++++++++ mm/filemap.c | 13 +++++++++---- mm/memory.c | 3 ++- 3 files changed, 19 insertions(+), 5 deletions(-)
include/linux/migrate.h +8 -0
@@ -97,6 +97,14 @@ static inline int set_movable_ops(const struct movable_operations *ops, enum pag return -ENOSYS; } +static inline void migration_entry_wait_on_locked (swp_entry_t entry,spinlock_t *ptl) +__releases (ptl) +{ +WARN_ON_ONCE (1 ); + +spin_unlock (ptl); +} +#endif /* CONFIG_MIGRATION */ #ifdef CONFIG_NUMA_BALANCING
mm/filemap.c +9 -4
@@ -1386,14 +1386,16 @@ static inline int folio_wait_bit_common(struct folio *folio, int bit_nr,#ifdef CONFIG_MIGRATION /** -* migration_entry_wait_on_locked - Wait for a migration entry to be removed -* @entry: migration swap entry. +* migration_entry_wait_on_locked - Wait for a migration entry or +* device_private entry to be removed. +* @entry: migration or device_private swap entry. * @ptl: already locked ptl. This function will drop the lock. * -* Wait for a migration entry referencing the given page to be removed. This is +* Wait for a migration entry referencing the given page, or device_private +* entry referencing a dvice_private page to be unlocked. This is * equivalent to folio_put_wait_locked(folio, TASK_UNINTERRUPTIBLE) except * this can be called without taking a reference on the page. Instead this -* should be called while holding the ptl for the migration entry referencing +* should be called while holding the ptl for @entry referencing * the page. * * Returns after unlocking the ptl.
@@ -1435,6 +1437,9 @@ void migration_entry_wait_on_locked(swp_entry_t entry, spinlock_t *ptl)* If a migration entry exists for the page the migration path must hold * a valid reference to the page, and it must take the ptl to remove the * migration entry. So the page is valid until the ptl is dropped. +* Similarly any path attempting to drop the last reference to a +* device-private page needs to grab the ptl to remove the device-private +* entry. */ spin_unlock (ptl);
mm/memory.c +2 -1
@@ -4642,7 +4642,8 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)unlock_page (vmf->page);put_page (vmf->page); }else { -pte_unmap_unlock (vmf->pte, vmf->ptl); +pte_unmap (vmf->pte); +migration_entry_wait_on_locked (entry, vmf->ptl); } }else if (is_hwpoison_entry (entry)) { ret =VM_FAULT_HWPOISON ;
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2.53.0