On Fri 02-02-24 09:42:27, Yang Shi wrote: > But if the partial range is MADV_FREE, khugepaged won't skip them. > This is what your second test case does. > > Secondly, I think it depends on the semantics of MADV_FREE, > particularly how to treat the redirtied pages. TBH I'm always confused > by the semantics. For example, the page contained "abcd", then it was > MADV_FREE'ed, then it was written again with "1234" after "abcd". So > the user should expect to see "abcd1234" or "00001234". Correct. You cannot assume the content of the first page as it could have been reclaimed at any time. > I'm supposed it should be "abcd1234" since MADV_FREE pages are still > valid and available, if I'm wrong please feel free to correct me. If > so we should always copy MADV_FREE pages in khugepaged regardless of > whether it is redirtied or not otherwise it may incur data corruption. > If we don't copy, then the follow up redirty after collapse to the > hugepage may return "00001234", right? Right. As pointed above this is a valid outcome if the page has been dropped. User has means to tell that from /proc/vmstat though. Not in a great precision but I think it would be really surprising to not see any pglazyfreed yet the content is gone. I think it would be legit to call it a bug. One could argue the bug would be in the accounting rather than the khugepaged implementation because madvised pages could be dropped at any time. But I think it makes more sense to copy the existing content. -- Michal Hocko SUSE Labs
