On Wed, 21 Oct 2015, Hugh Dickins wrote: > On Thu, 22 Oct 2015, Minchan Kim wrote: > > Hello Hugh, > > > > On Wed, Oct 21, 2015 at 05:59:59PM -0700, Hugh Dickins wrote: > > > On Thu, 22 Oct 2015, Minchan Kim wrote: > > > > > > > > I added the code to check it and queued it again but I had another oops > > > > in this time but symptom is related to anon_vma, too. > > > > (kernel is based on recent mmotm + unconditional mkdirty for bug fix) > > > > It seems page_get_anon_vma returns NULL since the page was not > > > > page_mapped > > > > at that time but second check of page_mapped right before try_to_unmap > > > > seems > > > > to be true. > > > > > > > > Adding 4191228k swap on /dev/vda5. Priority:-1 extents:1 > > > > across:4191228k FS > > > > Adding 4191228k swap on /dev/vda5. Priority:-1 extents:1 > > > > across:4191228k FS > > > > page:ffffea0001cfbfc0 count:3 mapcount:1 mapping:ffff88007f1b5f51 > > > > index:0x600000aff > > > > flags: 0x4000000000048019(locked|uptodate|dirty|swapcache|swapbacked) > > > > page dumped because: VM_BUG_ON_PAGE(PageAnon(page) && !PageKsm(page) && > > > > !anon_vma) > > > > > > That's interesting, that's one I added in my page migration series. > > > Let me think on it, but it could well relate to the one you got before.
I think I have introduced a bug there; or rather, made more evident a pre-existing bug. But I'm not sure yet: the stacktrace was from compaction (called by khugepaged, but that may not be relevant at all), and thinking through the races with isolate_migratepages_block() is never easy. What's certain is that I was not giving any thought to isolate_migratepages_block() when I added that VM_BUG_ON_PAGE(): I was thinking about "stable" anonymous pages, and how they get faulted back in from swapcache while holding page lock. It looks to me now as if a page might not yet be PageAnon when it's first tested in __unmap_and_move(), when going to page_get_anon_vma(); but is page_mapped() and PageAnon() by time of calling try_to_unmap(), where I inserted the VM_BUG_ON_PAGE(). If so, the code would always have been wrong (trying to unmap the anonymous page, and later remap its replacement, without a hold on the anon_vma needed to guide both lookups); but I'll have made it more glaringly wrong with the VM_BUG_ON_PAGE() - let me pretend that's a good step forward :) There's a reference count check in isolated_migratepages_block() before this, which would make it unlikely, but I doubt rules it out. However... you did hit an anon_vma reference counting problem before my migration changes went in, and Kirill had a vague suspicion that he might be screwing up anon_vma refcounting in split_huge_page(): if he confirms that, I'd say it's more likely to be the cause of your crash on this occasion. Not hard to fix mine (though we'll probably have to lose the VM_BUG_ON_PAGE on the way, so the real fix will be hidden by that trivial fix), I just want to give the races more thought. However it turns out, I think you have a very useful test there. (And I've observed no PageDirty problems with your recent patchsets, though I don't use MADV_FREE at all myself.) Hugh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
