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    SubjectRe: [PATCH] mm: thp: check total_mapcount instead of page_mapcount
    On Fri, Apr 30, 2021 at 3:30 PM Zi Yan <ziy@nvidia.com> wrote:
    >
    > On 30 Apr 2021, at 17:56, Yang Shi wrote:
    >
    > > On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <ziy@nvidia.com> wrote:
    > >>
    > >> On 30 Apr 2021, at 17:07, Yang Shi wrote:
    > >>
    > >>> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
    > >>> return false positive for PTE-mapped THP since page_mapcount() is used
    > >>> to check if the THP is unmapped, but it just checks compound mapount and
    > >>> head page's mapcount. If the THP is PTE-mapped and head page is not
    > >>> mapped, it may return false positive.
    > >>>
    > >>> Use total_mapcount() instead of page_mapcount() and do so for the
    > >>> VM_BUG_ON_PAGE in split_huge_page_to_list as well.
    > >>>
    > >>> [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/
    > >>>
    > >>> Signed-off-by: Yang Shi <shy828301@gmail.com>
    > >>> ---
    > >>> mm/huge_memory.c | 2 +-
    > >>> mm/rmap.c | 2 +-
    > >>> 2 files changed, 2 insertions(+), 2 deletions(-)
    > >>>
    > >>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    > >>> index 63ed6b25deaa..2122c3e853b9 100644
    > >>> --- a/mm/huge_memory.c
    > >>> +++ b/mm/huge_memory.c
    > >>> @@ -2718,7 +2718,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
    > >>> }
    > >>>
    > >>> unmap_page(head);
    > >>> - VM_BUG_ON_PAGE(compound_mapcount(head), head);
    > >>> + VM_BUG_ON_PAGE(total_mapcount(head), head);
    > >>
    > >> I am not sure about this change. The code below also checks total_mapcount(head)
    > >> and returns EBUSY if the count is non-zero. This change makes the code dead.
    > >
    > > It is actually dead if CONFIG_DEBUG_VM is enabled and total_mapcount
    > > is not 0 regardless of this change due to the below code, right?
    > >
    > > if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
    > > pr_alert("total_mapcount: %u, page_count(): %u\n",
    > > mapcount, count);
    > > if (PageTail(page))
    > > dump_page(head, NULL);
    > > dump_page(page, "total_mapcount(head) > 0");
    > > BUG();
    > > }
    >
    > Right. But with this change, mapcount will never be non-zero. The code above
    > will be useless and can be removed.

    Yes, you are correct.

    >
    > >> On the other hand, the change will force all mappings to the page have to be
    > >> successfully unmapped all the time. I am not sure if we want to do that.
    > >> Maybe it is better to just check total_mapcount() and fail the split.
    > >> The same situation happens with the code change below.
    > >
    > > IIUC, the code did force all mappings to the page to be unmapped in
    > > order to split it.
    > >>
    > >>>
    > >>> /* block interrupt reentry in xa_lock and spinlock */
    > >>> local_irq_disable();
    > >>> diff --git a/mm/rmap.c b/mm/rmap.c
    > >>> index 693a610e181d..2e547378ab5f 100644
    > >>> --- a/mm/rmap.c
    > >>> +++ b/mm/rmap.c
    > >>> @@ -1777,7 +1777,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
    > >>> else
    > >>> rmap_walk(page, &rwc);
    > >>>
    > >>> - return !page_mapcount(page) ? true : false;
    > >>> + return !total_mapcount(page) ? true : false;
    > >>> }
    > >>
    > >> In unmap_page(), VM_BUG_ON_PAGE(!unmap_success, page) will force all mappings
    > >> to the page have to be all unmapped, which might not be the case we want.
    > >
    > > AFAICT, I don't see such a case from all the callers of
    > > try_to_unmap(). Imay miss something, but I do have a hard time
    > > thinking of a usecase which can proceed safely with "not fully
    > > unmapped" page.
    >
    > This code change is correct, but after the change unmap_page() will fire VM_BUG_ON
    > when not all mappings are unmapped. Along with the change above, we will have
    > two identical VM_BUG_ONs happen one after another. We might want to remove one
    > of them.

    Yes. I'd prefer keep the one after unmap_page() since it seems more
    obvious. Any objection?

    >
    > Also, this changes the semantics of try_to_unmap. The comment for try_to_unmap
    > might need to be updated.

    What comment do you refer to?

    >
    >
    > —
    > Best Regards,
    > Yan Zi

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