Re: [PATCH 09/10] mm/hugetlb: Make page_vma_mapped_walk() safe to pmd unshare

Mike Kravetz
2022-12-05T23:54:47Z

On 11/29/22 14:35, Peter Xu wrote:
> Since page_vma_mapped_walk() walks the pgtable, it needs the vma lock
> to make sure the pgtable page will not be freed concurrently.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
> include/linux/rmap.h | 4 ++++
> mm/page_vma_mapped.c | 5 ++++-
> 2 files changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index bd3504d11b15..a50d18bb86aa 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -13,6 +13,7 @@
> #include <linux/highmem.h>
> #include <linux/pagemap.h>
> #include <linux/memremap.h>
> +#include <linux/hugetlb.h>
>
> /*
> * The anon_vma heads a list of private "related" vmas, to scan if
> @@ -408,6 +409,9 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
> pte_unmap(pvmw->pte);
> if (pvmw->ptl)
> spin_unlock(pvmw->ptl);
> + /* This needs to be after unlock of the spinlock */
> + if (is_vm_hugetlb_page(pvmw->vma))
> + hugetlb_vma_unlock_read(pvmw->vma);
> }
>
> bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index 93e13fc17d3c..f94ec78b54ff 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -169,10 +169,13 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> if (pvmw->pte)
> return not_found(pvmw);
>
> + hugetlb_vma_lock_read(vma);
> /* when pud is not present, pte will be NULL */
> pvmw->pte = huge_pte_offset(mm, pvmw->address, size);
> - if (!pvmw->pte)
> + if (!pvmw->pte) {
> + hugetlb_vma_unlock_read(vma);
> return false;
> + }
>
> pvmw->ptl = huge_pte_lock(hstate, mm, pvmw->pte);
> if (!check_pte(pvmw))

I think this is going to cause try_to_unmap() to always fail for hugetlb
shared pages. See try_to_unmap_one:

while (page_vma_mapped_walk(&pvmw)) {
...
if (folio_test_hugetlb(folio)) {
...
/*
* To call huge_pmd_unshare, i_mmap_rwsem must be
* held in write mode. Caller needs to explicitly
* do this outside rmap routines.
*
* We also must hold hugetlb vma_lock in write mode.
* Lock order dictates acquiring vma_lock BEFORE
* i_mmap_rwsem. We can only try lock here and fail
* if unsuccessful.
*/
if (!anon) {
VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
if (!hugetlb_vma_trylock_write(vma)) {
page_vma_mapped_walk_done(&pvmw);
ret = false;
}

Can not think of a great solution right now.
--
Mike Kravetz