| Message ID | c7b3ba9e8e58efe7bba9d52ce30a8ff4f4298e2f.1727148662.git.zhengqi.arch@bytedance.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | introduce pte_offset_map_{ro|rw}_nolock() | expand |
> On Sep 24, 2024, at 14:10, Qi Zheng <zhengqi.arch@bytedance.com> wrote: > > In move_ptes(), we may modify the new_pte after acquiring the new_ptl, so > convert it to using pte_offset_map_rw_nolock(). Now new_pte is none, so > hpage_collapse_scan_file() path can not find this by traversing > file->f_mapping, so there is no concurrency with retract_page_tables(). In > addition, we already hold the exclusive mmap_lock, so this new_pte page is > stable, so there is no need to get pmdval and do pmd_same() check. > > Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> Reviewed-by: Muchun Song <muchun.song@linux.dev>
diff --git a/mm/mremap.c b/mm/mremap.c index 24712f8dbb6b5..9dffd4a5b4d18 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -143,6 +143,7 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, spinlock_t *old_ptl, *new_ptl; bool force_flush = false; unsigned long len = old_end - old_addr; + pmd_t dummy_pmdval; int err = 0; /* @@ -175,7 +176,15 @@ static int move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, err = -EAGAIN; goto out; } - new_pte = pte_offset_map_nolock(mm, new_pmd, new_addr, &new_ptl); + /* + * Now new_pte is none, so hpage_collapse_scan_file() path can not find + * this by traversing file->f_mapping, so there is no concurrency with + * retract_page_tables(). In addition, we already hold the exclusive + * mmap_lock, so this new_pte page is stable, so there is no need to get + * pmdval and do pmd_same() check. + */ + new_pte = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval, + &new_ptl); if (!new_pte) { pte_unmap_unlock(old_pte, old_ptl); err = -EAGAIN;
In move_ptes(), we may modify the new_pte after acquiring the new_ptl, so convert it to using pte_offset_map_rw_nolock(). Now new_pte is none, so hpage_collapse_scan_file() path can not find this by traversing file->f_mapping, so there is no concurrency with retract_page_tables(). In addition, we already hold the exclusive mmap_lock, so this new_pte page is stable, so there is no need to get pmdval and do pmd_same() check. Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> --- mm/mremap.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-)