Messages in this thread | | | Subject | Re: [PATCH] mm: khugepaged: fix potential page state corruption | From | Yang Shi <> | Date | Thu, 19 Mar 2020 09:57:47 -0700 |
| |
On 3/19/20 3:49 AM, Kirill A. Shutemov wrote: > On Wed, Mar 18, 2020 at 10:39:21PM -0700, Yang Shi wrote: >> >> On 3/18/20 5:55 PM, Yang Shi wrote: >>> >>> On 3/18/20 5:12 PM, Kirill A. Shutemov wrote: >>>> On Thu, Mar 19, 2020 at 07:19:42AM +0800, Yang Shi wrote: >>>>> When khugepaged collapses anonymous pages, the base pages would >>>>> be freed >>>>> via pagevec or free_page_and_swap_cache(). But, the anonymous page may >>>>> be added back to LRU, then it might result in the below race: >>>>> >>>>> CPU A CPU B >>>>> khugepaged: >>>>> unlock page >>>>> putback_lru_page >>>>> add to lru >>>>> page reclaim: >>>>> isolate this page >>>>> try_to_unmap >>>>> page_remove_rmap <-- corrupt _mapcount >>>>> >>>>> It looks nothing would prevent the pages from isolating by reclaimer. >>>> Hm. Why should it? >>>> >>>> try_to_unmap() doesn't exclude parallel page unmapping. _mapcount is >>>> protected by ptl. And this particular _mapcount pin is reachable for >>>> reclaim as it's not part of usual page table tree. Basically >>>> try_to_unmap() will never succeeds until we give up the _mapcount on >>>> khugepaged side. >>> I don't quite get. What does "not part of usual page table tree" means? >>> >>> How's about try_to_unmap() acquires ptl before khugepaged? > The page table we are dealing with was detached from the process' page > table tree: see pmdp_collapse_flush(). try_to_unmap() will not see the > pte. > > try_to_unmap() can only reach the ptl if split ptl is disabled > (mm->page_table_lock is used), but it still will not be able to reach pte.
Aha, got it. Thanks for explaining. I definitely missed this point. Yes, pmdp_collapse_flush() would clear the pmd, then others won't see the page table.
However, it looks the vmscan would not stop at try_to_unmap() at all, try_to_unmap() would just return true since pmd_present() should return false in pvmw. Then it would go all the way down to __remove_mapping(), but freezing the page would fail since try_to_unmap() doesn't actually drop the refcount from the pte map.
It would not result in any critical problem AFAICT, but suboptimal and it may causes some unnecessary I/O due to swap.
> >>>> I don't see the issue right away. >>>> >>>>> The other problem is the page's active or unevictable flag might be >>>>> still set when freeing the page via free_page_and_swap_cache(). >>>> So what? >>> The flags may leak to page free path then kernel may complain if >>> DEBUG_VM is set. > Could you elaborate on what codepath you are talking about?
__put_page -> __put_single_page -> free_unref_page -> put_unref_page_prepare -> free_pcp_prepare -> free_pages_prepare -> free_pages_check
This check would just be run when DEBUG_VM is enabled.
> >>>>> The putback_lru_page() would not clear those two flags if the pages are >>>>> released via pagevec, it sounds nothing prevents from isolating active >> Sorry, this is a typo. If the page is freed via pagevec, active and >> unevictable flag would get cleared before freeing by page_off_lru(). >> >> But, if the page is freed by free_page_and_swap_cache(), these two flags are >> not cleared. But, it seems this path is hit rare, the pages are freed by >> pagevec for the most cases. >> >>>>> or unevictable pages. >>>> Again, why should it? vmscan is equipped to deal with this. >>> I don't mean vmscan, I mean khugepaged may isolate active and >>> unevictable pages since it just simply walks page table. > Why it is wrong? lru_cache_add() only complains if both flags set, it > shouldn't happen.
Noting wrong about isolating active or unevictable pages. I just mean it seems possible active or unevictable flag may be there if the page is freed via free_page_add_swap_cache() path.
> >>>>> However I didn't really run into these problems, just in theory >>>>> by visual >>>>> inspection. >>>>> >>>>> And, it also seems unnecessary to have the pages add back to LRU >>>>> again since >>>>> they are about to be freed when reaching this point. So, >>>>> clearing active >>>>> and unevictable flags, unlocking and dropping refcount from isolate >>>>> instead of calling putback_lru_page() as what page cache collapse does. >>>> Hm? But we do call putback_lru_page() on the way out. I do not follow. >>> It just calls putback_lru_page() at error path, not success path. >>> Putting pages back to lru on error path definitely makes sense. Here it >>> is the success path. > I agree that putting the apage on LRU just before free the page is > suboptimal, but I don't see it as a critical issue.
Yes, given the code analysis above, I agree. If you thought the patch is a fine micro-optimization, I would like to re-submit it with rectified commit log. Thank you for your time.
> >
| |