> On 4 Jun 2024, at 0:57, David Hildenbrand wrote:
>
> > On 04.06.24 07:47, xu.xin16@xxxxxxxxxx wrote:
> >> From: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> >>
> >> When I did a large folios split test, a WARNING
> >> "[ 5059.122759][ T166] Cannot split file folio to non-0 order"
> >> was triggered. But my test cases are only for anonmous folios.
> >> while mapping_large_folio_support() is only reasonable for page
> >> cache folios.
> >
> > Agreed.
> >
> > I wonder if mapping_large_folio_support() should either
> >
> > a) Complain if used for anon folios, so we can detect the wrong use more easily. (VM_WARN_ON_ONCE())
>
> This is much better.
>
> >
> > b) Return "true" for anonymous mappings, although that's more debatable.
>
> This might fix the warning here, but the function might get wrong uses easily.
yes, maybe we should rename mapping_large_folio_support() if we choose b).
> >
> >>
> >> In split_huge_page_to_list_to_order(), the folio passed to
> >> mapping_large_folio_support() maybe anonmous folio. The
> >> folio_test_anon() check is missing. So the split of the anonmous THP
> >> is failed. This is also the same for shmem_mapping(). We'd better add
> >> a check for both. But the shmem_mapping() in __split_huge_page() is
> >> not involved, as for anonmous folios, the end parameter is set to -1, so
> >> (head[i].index >= end) is always false. shmem_mapping() is not called.
> >>
> >> Using /sys/kernel/debug/split_huge_pages to verify this, with this
> >> patch, large anon THP is successfully split and the warning is ceased.
> >>
> >> Signed-off-by: Ran Xiaokai <ran.xiaokai@xxxxxxxxxx>
> >> Cc: xu xin <xu.xin16@xxxxxxxxxx>
> >> Cc: Yang Yang <yang.yang29@xxxxxxxxxx>
> >> ---
> >> mm/huge_memory.c | 38 ++++++++++++++++++++------------------
> >> 1 file changed, 20 insertions(+), 18 deletions(-)
> >>
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 317de2afd371..4c9c7e5ea20c 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -3009,31 +3009,33 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >> if (new_order >= folio_order(folio))
> >> return -EINVAL;
> >>
> >> - /* Cannot split anonymous THP to order-1 */
> >> - if (new_order == 1 && folio_test_anon(folio)) {
> >> - VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> >> - return -EINVAL;
> >> - }
> >> -
> >> if (new_order) {
> >> /* Only swapping a whole PMD-mapped folio is supported */
> >> if (folio_test_swapcache(folio))
> >> return -EINVAL;
> >> - /* Split shmem folio to non-zero order not supported */
> >> - if (shmem_mapping(folio->mapping)) {
> >> - VM_WARN_ONCE(1,
> >> - "Cannot split shmem folio to non-0 order");
> >> - return -EINVAL;
> >> - }
> >> - /* No split if the file system does not support large folio */
> >> - if (!mapping_large_folio_support(folio->mapping)) {
> >> - VM_WARN_ONCE(1,
> >> - "Cannot split file folio to non-0 order");
> >> - return -EINVAL;
> >> +
> >> + if (folio_test_anon(folio)) {
> >> + /* Cannot split anonymous THP to order-1 */
> >> + if (new_order == 1) {
> >> + VM_WARN_ONCE(1, "Cannot split to order-1 folio");
> >> + return -EINVAL;
> >> + }
> >> + } else {
> >> + /* Split shmem folio to non-zero order not supported */
> >> + if (shmem_mapping(folio->mapping)) {
> >> + VM_WARN_ONCE(1,
> >> + "Cannot split shmem folio to non-0 order");
> >> + return -EINVAL;
> >> + }
> >> + /* No split if the file system does not support large folio */
> >> + if (!mapping_large_folio_support(folio->mapping)) {
> >> + VM_WARN_ONCE(1,
> >> + "Cannot split file folio to non-0 order");
> >> + return -EINVAL;
> >> + }
> >> }
> >> }
> >
> > What about the following sequence:
> >
> > if (folio_test_anon(folio)) {
> > if (new_order == 1)
> > ...
> > } else if (new_order) {
> > if (shmem_mapping(...))
> > ...
> > ...
> > }
> >
> > if (folio_test_swapcache(folio) && new_order)
> > return -EINVAL;
> >
> > Should result in less churn and reduce indentation level.
>
> Yeah, this looks better to me.
>
> Best Regards,
> Yan, Zi
