On Fri, Apr 30, 2021 at 3:30 PM Zi Yan <ziy@xxxxxxxxxx> wrote: > > On 30 Apr 2021, at 17:56, Yang Shi wrote: > > > On Fri, Apr 30, 2021 at 2:30 PM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxxxx/ > >>> > >>> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> > >>> --- > >>> 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
