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. >> 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. Also, this changes the semantics of try_to_unmap. The comment for try_to_unmap might need to be updated. — Best Regards, Yan Zi
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