Hello Kirill, On Mon, Jun 19, 2017 at 05:03:23PM +0300, Kirill A. Shutemov wrote: > On Fri, Jun 16, 2017 at 11:53:33PM +0900, Minchan Kim wrote: > > Hi Andrea, > > > > On Fri, Jun 16, 2017 at 04:27:20PM +0200, Andrea Arcangeli wrote: > > > Hello Minchan, > > > > > > On Fri, Jun 16, 2017 at 10:52:09PM +0900, Minchan Kim wrote: > > > > > > > @@ -1995,8 +1984,6 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > > > > > > > if (soft_dirty) > > > > > > > entry = pte_mksoft_dirty(entry); > > > > > > > } > > > > > > > - if (dirty) > > > > > > > - SetPageDirty(page + i); > > > > > > > pte = pte_offset_map(&_pmd, addr); > > > [..] > > > > > > > > split_huge_page set PG_dirty to all subpages unconditionally? > > > > If it's true, yes, it doesn't break MADV_FREE. However, I didn't spot > > > > that piece of code. What I found one is just __split_huge_page_tail > > > > which set PG_dirty to subpage if head page is dirty. IOW, if the head > > > > page is not dirty, tail page will be clean, too. > > > > Could you point out what routine set PG_dirty to all subpages unconditionally? > > When I wrote this code, I considered that we may want to track dirty > status on per-4k basis for file-backed THPs. > > > > On a side note the snippet deleted above was useless, as long as > > > there's one left hugepmd to split, the physical page has to be still > > > compound and huge and as long as that's the case the tail pages > > > PG_dirty bit is meaningless (even if set, it's going to be clobbered > > > during the physical split). > > > > I got it during reviewing this patch. That's why I didn't argue > > this patch would break MADV_FREE by deleting routine which propagate > > dirty to pte of subpages. However, although it's useless, I prefer > > not removing the transfer of dirty bit. Because it would help MADV_FREE > > users who want to use smaps to know how many of pages are not freeable > > (i.e, dirtied) since MADV_FREE although it is not 100% correct. > > > > > > > > In short PG_dirty is only meaningful in the head as long as it's > > > compound. The physical split in __split_huge_page_tail transfer the > > > head value to the tails like you mentioned, that's all as far as I can > > > tell. > > > > Thanks for the comment. Then, this patch is to fix MADV_FREE's bug > > which has lost dirty bit by transferring dirty bit too early. > > Erghh. I've misread splitting code. Yes, it's not unconditional. So we fix > actual bug. > > But I'm not sure it's subject for -stable. I haven't seen any bug reports > that can be attributed to the bug. Okay, I'm not against but please rewrite changelog to indicate it fixes the problem. One more thing, as I mentioned, I don't want to remove pmd dirty bit -> PG_dirty propagate to subpage part because it would be helpful for MADV_FREE users. Thanks. > > -- > Kirill A. Shutemov
