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. 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. > > /* 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. 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
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