[PATCH RFC v2 10/12] mm/hugetlb: Use hugetlb walker lock in walk_hugetlb_range()

Peter Xu
2022-11-18T01:12:59Z

Hugetlb walker lock makes sure the pte_t* won't go away from under us. One
thing to mention is there're two hugetlb_entry() users that can yield the
thread within hugetlb_entry(), that'll need to add unlock/lock pair around
the yield, meanwhile document hugetlb_entry() explaining the lock for
sleepable hugetlb_entry()s.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
arch/s390/mm/gmap.c | 2 ++
fs/proc/task_mmu.c | 2 ++
include/linux/pagewalk.h | 9 ++++++++-
mm/pagewalk.c | 2 ++
4 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index 02d15c8dc92e..fb2938e8d1c7 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -2644,7 +2644,9 @@ static int __s390_enable_skey_hugetlb(pte_t *pte, unsigned long addr,
end = start + HPAGE_SIZE - 1;
__storage_key_init_range(start, end);
set_bit(PG_arch_1, &page->flags);
+ hugetlb_walker_unlock();
cond_resched();
+ hugetlb_walker_lock();
return 0;
}

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 89338950afd3..ed750a52e60b 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1612,7 +1612,9 @@ static int pagemap_hugetlb_range(pte_t *ptep, unsigned long hmask,
frame++;
}

+ hugetlb_walker_unlock();
cond_resched();
+ hugetlb_walker_lock();

return err;
}
diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 959f52e5867d..7fa3724c6eb5 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -21,7 +21,14 @@ struct mm_walk;
* depth is -1 if not known, 0:PGD, 1:P4D, 2:PUD, 3:PMD.
* Any folded depths (where PTRS_PER_P?D is equal to 1)
* are skipped.
- * @hugetlb_entry: if set, called for each hugetlb entry
+ * @hugetlb_entry: if set, called for each hugetlb entry. Note that
+ * each pte_t* is protected by hugetlb_walker_lock(),
+ * and the lock does not allow sleep. If explicit
+ * sleep in the entry fn needed, the caller needs to
+ * release the lock (hugetlb_walker_unlock()), then
+ * relock it (hugetlb_walker_lock()) before return.
+ * After the unlock, the pte_t* may become invalid
+ * anytime so cannot be accessed anymore.
* @test_walk: caller specific callback function to determine whether
* we walk over the current vma or not. Returning 0 means
* "do page table walk over the current vma", returning
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index 7f1c9b274906..abf310011ab1 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -302,6 +302,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
const struct mm_walk_ops *ops = walk->ops;
int err = 0;

+ hugetlb_walker_lock();
do {
next = hugetlb_entry_end(h, addr, end);
pte = huge_pte_offset(walk->mm, addr & hmask, sz);
@@ -314,6 +315,7 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end,
if (err)
break;
} while (addr = next, addr != end);
+ hugetlb_walker_unlock();

return err;
}
--
2.37.3