Messages in this thread | | | Subject | Re: [f2fs-dev] [PATCH] f2fs: initialize page->private when using for our internal use | From | Chao Yu <> | Date | Tue, 13 Jul 2021 08:46:37 +0800 |
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On 2021/7/12 14:53, Michal Hocko wrote: > On Sat 10-07-21 16:11:38, Chao Yu wrote: >> On 2021/7/7 17:57, Mel Gorman wrote: >>> I think it would work but it would be preferable to find out why the >>> tail page has an order set in the first place. I've looked over >> >> Agreed. >> >>> mm/page_alloc.c and mm/compaction.c a few times and did not spot where >>> set_private_page(page, 0) is missed when it should be covered by >>> clear_page_guard or del_page_from_free_list :( >> >> I didn't enable CONFIG_DEBUG_PAGEALLOC, so we will expect page private >> should be cleared by del_page_from_free_list(), but I guess it only clears >> the buddy's private field rather than original page's, so I added below >> diff and check the dmesg, it looks stall private value in original page >> will be left commonly... Let me know if I missed something? > > Page private should be cleared when the page is freed to the allocator. > Have a look at PAGE_FLAGS_CHECK_AT_FREE.
Quoted from Jaegeuk's comments in [1]
"Hmm, I can see it in 4.14 and 5.10 kernel.
The trace is on:
30875 [ 1065.118750] c3 87 f2fs_migrate_page+0x354/0x45c 30876 [ 1065.123872] c3 87 move_to_new_page+0x70/0x30c 30877 [ 1065.128813] c3 87 migrate_pages+0x3a0/0x964 30878 [ 1065.133583] c3 87 compact_zone+0x608/0xb04 30879 [ 1065.138257] c3 87 kcompactd+0x378/0x4ec 30880 [ 1065.142664] c3 87 kthread+0x11c/0x12c 30881 [ 1065.146897] c3 87 ret_from_fork+0x10/0x18
It seems compaction_alloc() gets a free page which doesn't reset the fields?"
https://lore.kernel.org/linux-f2fs-devel/YOvm2faBUjKmZI7Q@dhcp22.suse.cz/T/#m98a4a5e777f5b0e7366b367463efafd2133dd681
So problem here we met is: in f2fs_migrate_page(), newpage may has stall .private value rather than PG_private flag, which may cause f2fs will treat the page with wrong private status.
> >> --- >> mm/page_alloc.c | 5 +++++ >> 1 file changed, 5 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index a06bcfe6f786..1e7031ff548e 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -1029,6 +1029,7 @@ static inline void __free_one_page(struct page *page, >> unsigned long combined_pfn; >> unsigned int max_order; >> struct page *buddy; >> + struct page *orig_page = page; >> bool to_tail; >> >> max_order = min_t(unsigned int, MAX_ORDER - 1, pageblock_order); >> @@ -1097,6 +1098,10 @@ static inline void __free_one_page(struct page *page, >> >> done_merging: >> set_buddy_order(page, order); >> + if (orig_page != page) { >> + if (WARN_ON_ONCE(orig_page->private)) >> + pr_info("2order:%x, origpage.private:%x", order, orig_page->private); >> + } > > Why is this expected? Buddy allocator uses page private to store order. > Whether we are merging to the freed page or coalesce it to a different
The order was only set in head page, right? Since it looks __free_one_page() tries to clear page.private for every buddy with del_page_from_free_list().
If that is true, after done_merging label in __free_one_page, if original page is a tail page, we may missed to clear its page.private field?
Thanks,
> page is not all that important. >
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