[PATCH v2 24/32] mm/migrate_device: allow pte_offset_map_lock() to fail

Hugh Dickins
2023-06-09T01:38:28Z

migrate_vma_collect_pmd(): remove the pmd_trans_unstable() handling after
splitting huge zero pmd, and the pmd_none() handling after successfully
splitting huge page: those are now managed inside pte_offset_map_lock(),
and by "goto again" when it fails.

But the skip after unsuccessful split_huge_page() must stay: it avoids an
endless loop. The skip when pmd_bad()? Remove that: it will be treated
as a hole rather than a skip once cleared by pte_offset_map_lock(), but
with different timing that would be so anyway; and it's arguably best to
leave the pmd_bad() handling centralized there.

migrate_vma_insert_page(): remove comment on the old pte_offset_map()
and old locking limitations; remove the pmd_trans_unstable() check and
just proceed to pte_offset_map_lock(), aborting when it fails (page has
been charged to memcg, but as in other cases, it's uncharged when freed).

Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
---
mm/migrate_device.c | 31 ++++---------------------------
1 file changed, 4 insertions(+), 27 deletions(-)

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index d30c9de60b0d..a14af6b12b04 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -83,9 +83,6 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
if (is_huge_zero_page(page)) {
spin_unlock(ptl);
split_huge_pmd(vma, pmdp, addr);
- if (pmd_trans_unstable(pmdp))
- return migrate_vma_collect_skip(start, end,
- walk);
} else {
int ret;

@@ -100,16 +97,12 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
if (ret)
return migrate_vma_collect_skip(start, end,
walk);
- if (pmd_none(*pmdp))
- return migrate_vma_collect_hole(start, end, -1,
- walk);
}
}

- if (unlikely(pmd_bad(*pmdp)))
- return migrate_vma_collect_skip(start, end, walk);
-
ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
+ if (!ptep)
+ goto again;
arch_enter_lazy_mmu_mode();

for (; addr < end; addr += PAGE_SIZE, ptep++) {
@@ -595,27 +588,10 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
pmdp = pmd_alloc(mm, pudp, addr);
if (!pmdp)
goto abort;
-
if (pmd_trans_huge(*pmdp) || pmd_devmap(*pmdp))
goto abort;
-
- /*
- * Use pte_alloc() instead of pte_alloc_map(). We can't run
- * pte_offset_map() on pmds where a huge pmd might be created
- * from a different thread.
- *
- * pte_alloc_map() is safe to use under mmap_write_lock(mm) or when
- * parallel threads are excluded by other means.
- *
- * Here we only have mmap_read_lock(mm).
- */
if (pte_alloc(mm, pmdp))
goto abort;
-
- /* See the comment in pte_alloc_one_map() */
- if (unlikely(pmd_trans_unstable(pmdp)))
- goto abort;
-
if (unlikely(anon_vma_prepare(vma)))
goto abort;
if (mem_cgroup_charge(page_folio(page), vma->vm_mm, GFP_KERNEL))
@@ -650,7 +626,8 @@ static void migrate_vma_insert_page(struct migrate_vma *migrate,
}

ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl);
-
+ if (!ptep)
+ goto abort;
if (check_stable_address_space(mm))
goto unlock_abort;

--
2.35.3