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(); } > 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. > Maybe you will want to remove the VM_BUG_ON_PAGE here, check total_mapcount() > above, and fail the split if not all mappings to the pages are unmapped. > > > > — > Best Regards, > Yan Zi
