On Tue, Aug 17, 2021 at 1:17 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> khugepaged's collapse_file() currently uses SGP_NOHUGE to tell
> shmem_getpage() not to try allocating a huge page, in the very unlikely
> event that a racing hole-punch removes the swapped or fallocated page as
> soon as i_pages lock is dropped.
>
> We want to consolidate shmem's huge decisions, removing SGP_HUGE and
> SGP_NOHUGE; but cannot quite persuade ourselves that it's okay to regress
> the protection in this case - Yang Shi points out that the huge page
> would remain indefinitely, charged to root instead of the intended memcg.
>
> collapse_file() should not even allocate a small page in this case: why
> proceed if someone is punching a hole? SGP_READ is almost the right flag
> here, except that it optimizes away from a fallocated page, with NULL to
> tell caller to fill with zeroes (like a hole); whereas collapse_file()'s
> sequence relies on using a cache page. Add SGP_NOALLOC just for this.
>
> There are too many consecutive "if (page"s there in shmem_getpage_gfp():
> group it better; and fix the outdated "bring it back from swap" comment.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> ---
> include/linux/shmem_fs.h | 1 +
> mm/khugepaged.c | 2 +-
> mm/shmem.c | 29 +++++++++++++++++------------
> 3 files changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 9b7f7ac52351..7d97b15a2f7a 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -94,6 +94,7 @@ extern unsigned long shmem_partial_swap_usage(struct address_space *mapping,
> /* Flag allocation requirements to shmem_getpage */
> enum sgp_type {
> SGP_READ, /* don't exceed i_size, don't allocate page */
> + SGP_NOALLOC, /* similar, but fail on hole or use fallocated page */
> SGP_CACHE, /* don't exceed i_size, may allocate page */
> SGP_NOHUGE, /* like SGP_CACHE, but no huge pages */
> SGP_HUGE, /* like SGP_CACHE, huge pages preferred */
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b0412be08fa2..045cc579f724 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1721,7 +1721,7 @@ static void collapse_file(struct mm_struct *mm,
> xas_unlock_irq(&xas);
> /* swap in or instantiate fallocated page */
> if (shmem_getpage(mapping->host, index, &page,
> - SGP_NOHUGE)) {
> + SGP_NOALLOC)) {
> result = SCAN_FAIL;
> goto xa_unlocked;
> }
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 740d48ef1eb5..226ac3a911e9 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1871,26 +1871,31 @@ static int shmem_getpage_gfp(struct inode *inode, pgoff_t index,
> return error;
> }
>
> - if (page)
> + if (page) {
> hindex = page->index;
> - if (page && sgp == SGP_WRITE)
> - mark_page_accessed(page);
> -
> - /* fallocated page? */
> - if (page && !PageUptodate(page)) {
> + if (sgp == SGP_WRITE)
> + mark_page_accessed(page);
> + if (PageUptodate(page))
> + goto out;
> + /* fallocated page */
> if (sgp != SGP_READ)
> goto clear;
> unlock_page(page);
> put_page(page);
> - page = NULL;
> - hindex = index;
> }
> - if (page || sgp == SGP_READ)
> - goto out;
>
> /*
> - * Fast cache lookup did not find it:
> - * bring it back from swap or allocate.
> + * SGP_READ: succeed on hole, with NULL page, letting caller zero.
> + * SGP_NOALLOC: fail on hole, with NULL page, letting caller fail.
> + */
> + *pagep = NULL;
> + if (sgp == SGP_READ)
> + return 0;
> + if (sgp == SGP_NOALLOC)
> + return -ENOENT;
> +
> + /*
> + * Fast cache lookup and swap lookup did not find it: allocate.
> */
>
> if (vma && userfaultfd_missing(vma)) {
> --
> 2.26.2
>
