On Mon, Mar 02, 2015 at 09:59:20AM +0800, Wang, Yalin wrote: > > -----Original Message----- > > From: Minchan Kim [mailto:minchan.kim@xxxxxxxxx] On Behalf Of Minchan Kim > > Sent: Saturday, February 28, 2015 9:50 PM > > To: Wang, Yalin > > Cc: Michal Hocko; Andrew Morton; linux-kernel@xxxxxxxxxxxxxxx; linux- > > mm@xxxxxxxxx; Rik van Riel; Johannes Weiner; Mel Gorman; Shaohua Li > > Subject: Re: [RFC] mm: change mm_advise_free to clear page dirty > > > > On Fri, Feb 27, 2015 at 10:37:14PM +0900, Minchan Kim wrote: > > > On Fri, Feb 27, 2015 at 03:50:29PM +0800, Wang, Yalin wrote: > > > > > -----Original Message----- > > > > > From: Minchan Kim [mailto:minchan.kim@xxxxxxxxx] On Behalf Of Minchan > > Kim > > > > > Sent: Friday, February 27, 2015 2:44 PM > > > > > To: Wang, Yalin > > > > > Cc: Michal Hocko; Andrew Morton; linux-kernel@xxxxxxxxxxxxxxx; linux- > > > > > mm@xxxxxxxxx; Rik van Riel; Johannes Weiner; Mel Gorman; Shaohua Li > > > > > Subject: Re: [RFC] mm: change mm_advise_free to clear page dirty > > > > > > > > > > On Fri, Feb 27, 2015 at 01:48:48PM +0800, Wang, Yalin wrote: > > > > > > > -----Original Message----- > > > > > > > From: Minchan Kim [mailto:minchan.kim@xxxxxxxxx] On Behalf Of > > Minchan > > > > > Kim > > > > > > > Sent: Friday, February 27, 2015 1:28 PM > > > > > > > To: Wang, Yalin > > > > > > > Cc: Michal Hocko; Andrew Morton; linux-kernel@xxxxxxxxxxxxxxx; > > linux- > > > > > > > mm@xxxxxxxxx; Rik van Riel; Johannes Weiner; Mel Gorman; Shaohua > > Li > > > > > > > Subject: Re: [RFC] mm: change mm_advise_free to clear page dirty > > > > > > > > > > > > > > Hello, > > > > > > > > > > > > > > On Fri, Feb 27, 2015 at 11:37:18AM +0800, Wang, Yalin wrote: > > > > > > > > This patch add ClearPageDirty() to clear AnonPage dirty flag, > > > > > > > > the Anonpage mapcount must be 1, so that this page is only used > > by > > > > > > > > the current process, not shared by other process like fork(). > > > > > > > > if not clear page dirty for this anon page, the page will never > > be > > > > > > > > treated as freeable. > > > > > > > > > > > > > > In case of anonymous page, it has PG_dirty when VM adds it to > > > > > > > swap cache and clear it in clear_page_dirty_for_io. That's why > > > > > > > I added ClearPageDirty if we found it in swapcache. > > > > > > > What case am I missing? It would be better to understand if you > > > > > > > describe specific scenario. > > > > > > > > > > > > > > Thanks. > > > > > > > > > > > > > > > > > > > > > > > Signed-off-by: Yalin Wang <yalin.wang@xxxxxxxxxxxxxx> > > > > > > > > --- > > > > > > > > mm/madvise.c | 15 +++++---------- > > > > > > > > 1 file changed, 5 insertions(+), 10 deletions(-) > > > > > > > > > > > > > > > > diff --git a/mm/madvise.c b/mm/madvise.c > > > > > > > > index 6d0fcb8..257925a 100644 > > > > > > > > --- a/mm/madvise.c > > > > > > > > +++ b/mm/madvise.c > > > > > > > > @@ -297,22 +297,17 @@ static int madvise_free_pte_range(pmd_t > > *pmd, > > > > > > > unsigned long addr, > > > > > > > > continue; > > > > > > > > > > > > > > > > page = vm_normal_page(vma, addr, ptent); > > > > > > > > - if (!page) > > > > > > > > + if (!page || !PageAnon(page) > > || !trylock_page(page)) > > > > > > > > continue; > > > > > > > > > > > > > > > > if (PageSwapCache(page)) { > > > > > > > > - if (!trylock_page(page)) > > > > > > > > + if (!try_to_free_swap(page)) > > > > > > > > continue; > > > > > > > > - > > > > > > > > - if (!try_to_free_swap(page)) { > > > > > > > > - unlock_page(page); > > > > > > > > - continue; > > > > > > > > - } > > > > > > > > - > > > > > > > > - ClearPageDirty(page); > > > > > > > > - unlock_page(page); > > > > > > > > } > > > > > > > > > > > > > > > > + if (page_mapcount(page) == 1) > > > > > > > > + ClearPageDirty(page); > > > > > > > > + unlock_page(page); > > > > > > > > /* > > > > > > > > * Some of architecture(ex, PPC) don't update TLB > > > > > > > > * with set_pte_at and tlb_remove_tlb_entry so for > > > > > > > > -- > > > > > > Yes, for page which is in SwapCache, it is correct, > > > > > > But for anon page which is not in SwapCache, it is always > > > > > > PageDirty(), so we should also clear dirty bit to make it freeable, > > > > > > > > > > No. Every anon page starts from !PageDirty and it has PG_dirty > > > > > only when it's addeded into swap cache. If vm_swap_full turns on, > > > > > a page in swap cache could have PG_dirty via try_to_free_swap again. > > > > > > > > mmm.. > > > > sometimes you can see an anon page PageDirty(), but it is not in > > swapcache, > > > > for example, handle_pte_fault()-->do_swap_page()-->try_to_free_swap(), > > > > at this time, the page is deleted from swapcache and is marked > > PageDirty(), > > > > > > That's what I missed. It's clear and would be simple patch so > > > could you send a patch to fix this issue with detailed description > > > like above? > > > > > > > > > > > > > > > > So, Do you have concern about swapped-out pages when MADV_FREE is > > > > > called? If so, please look at my patch. > > > > > > > > > > https://lkml.org/lkml/2015/2/25/43 > > > > > > > > > > It will zap the swapped out page. So, this is not a issue any more? > > > > > > > > > > > > > > > > > Another problem is that if an anon page is shared by more than one > > > > > process, > > > > > > This happened when fork(), the anon page will be copy on write, > > > > > > In this case, we should not clear page dirty, > > > > > > This is not correct for other process which don't call MADV_FREE > > syscall. > > > > > > > > > > You mean we shouldn't inherit MADV_FREE attribute? > > > > > Why? > > > > > > > > Is it correct behavior if code like this: > > > > > > > > Parent: > > > > ptr1 = malloc(len); > > > > memset(ptr1, 'a', len); > > > > fork(); > > > > if (I am parent) > > > > madvise_free(ptr1, len); > > > > > > > > child: > > > > sleep(10); > > > > parse_data(ptr1, len); // child may see zero, not 'a', > > > > // is it the right behavior that the programer want? > > > > > > > > Because child don't call madvise_free(), so it should see 'a', not zero > > page. > > > > Isn't it ? > > > > > > You're absolutely right. Thanks. > > > But I doubt your fix is best. Most of fork will do exec soonish so > > > it's not a good idea to make MADV_FREE void even though hinted pages > > > are shared when the syscall was called. > > > How about checking the page is shared or not in reclaim path? > > > If it is still shared, we shouldn't discard it. > > > > I got confused. With looking at copy_one_pte, it copys from src_pte > > and not clear dirty bit if it's not a shared mapping. > > If so, in your example, child pte has pte dirty bit on while parent > > has clean bit by madvise_free so that VM shouldn't discard the page. > > No? > > > It is not always true that src_pte has dirty bit. > For example , > If a page have been swap into swap partition, > The pte have become a swap entry, > Then a read fault happened, > The pte will be a page pte without dirty bit, > Am I ritht? Parent A, Child B 1. Page : swapped-out, A: !pte_present B: !pte_present, A or B does madvise_free It's not pte_present so madvise_free will be void 2. Page : swapped-in, PageSwapCache A: pte_clean by swapin-read fault B: !pte_present, A does madvise_free madvise_free will do try_to_free_swap but other process(ie, B) is still reference the swap slot so it will fail so madvise_free by A will be void. 3. Page: swappined-in, !PageSwapCache A: pte_clean by swapin-read fault B: !pte_clean by swapin-read fault, A does madvise_free Since the page is removed from swap cache, it has PG_dirty so we couldn't discard the page(ie, we should swap out, not discard). Is there any usecase I am missing? Anyway, relying on PageDirty for madvise_free rather than pte_dirty makes lots of confuse. Originally, PageDirty flag for anonymous page is for avoidng unnecessary swapout so I will cook a patch and send RFC. Thanks. -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>