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    SubjectRe: [Qestion] Hit a WARN_ON_ONCE in try_to_unmap_one when runing syzkaller
    On 2019/3/14 14:27, Naoya Horiguchi wrote:
    > Hi,
    >
    > On Wed, Mar 13, 2019 at 12:03:20AM +0800, zhong jiang wrote:
    > ...
    >> Minchan has changed the conditon check from BUG_ON to WARN_ON_ONCE in try_to_unmap_one.
    >> However, It is still an abnormal condition when PageSwapBacked is not equal to PageSwapCache.
    >>
    >> But Is there any case it will meet the conditon in the mainline.
    >>
    >> It is assumed that PageSwapBacked(page) is true in the anonymous page, This is to say, PageSwapcache
    >> is false. however, That is impossible because we will update the pte for hwpoison entry.
    >>
    >> Because page is locked , Its page flags should not be changed except for PageSwapBacked
    > try_to_unmap_one() from hwpoison_user_mappings() could reach the
    > WARN_ON_ONCE() only if TTU_IGNORE_HWPOISON is set, because PageHWPoison()
    > is set at the beginning of memory_failure().
    >
    > Clearing TTU_IGNORE_HWPOISON might happen on the following two paths:
    >
    > static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
    > int flags, struct page **hpagep)
    > {
    > ...
    >
    > if (PageSwapCache(p)) {
    > pr_err("Memory failure: %#lx: keeping poisoned page in swap cache\n",
    > pfn);
    > ttu |= TTU_IGNORE_HWPOISON;
    > }
    > ...
    >
    > mapping = page_mapping(hpage);
    > if (!(flags & MF_MUST_KILL) && !PageDirty(hpage) && mapping &&
    > mapping_cap_writeback_dirty(mapping)) {
    > if (page_mkclean(hpage)) {
    > SetPageDirty(hpage);
    > } else {
    > kill = 0;
    > ttu |= TTU_IGNORE_HWPOISON;
    > pr_info("Memory failure: %#lx: corrupted page was clean: dropped without side effects\n",
    > pfn);
    > }
    > }
    > ...
    >
    > unmap_success = try_to_unmap(hpage, ttu);
    > ...
    >
    > So either of the above "ttu |= TTU_IGNORE_HWPOISON" should be executed.
    > I'm not sure which one, but both paths show printk messages, so if you
    > could have kernel message log, that might help ...
    Thank you for your response.

    Unfortunately, I lost the printk log. I was looking for it before and support us for further analysis.

    It's very weird to get there. Assume that TTU_IGNORE_HWPOSISON is set. There is the two case.

    First, PageSwapCache is set and page has been locked. Theoretically WARN_ON_ONCE should not be triggered.
    Second, We should assume the page belongs to file page.:-(

    I will go on reproducing the issue and get the printk message log.

    Thanks
    zhong jiang
    > Thanks,
    > Naoya Horiguchi
    >
    > .
    >


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