On 30 Apr 2021, at 18:55, Yang Shi wrote: > 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? Sounds good to me. > >> >> 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? /** * try_to_unmap - try to remove all page table mappings to a page a page -> a page and the compound page it belongs to * @page: the page to get unmapped the page -> the page or the subpage of a compound page * @flags: action and flags * * Tries to remove all the page table entries which are mapping this * page, used in the pageout path. Caller must hold the page lock. this page -> this page and the compound page it belongs to Feel free to change the wording if you find better ones. — Best Regards, Yan Zi
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