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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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