On 2021/5/11 7:42, Hugh Dickins wrote: > On Mon, 10 May 2021, Miaohe Lin wrote: >> On 2021/5/10 13:59, Hugh Dickins wrote: >>> This reverts commit 3e96b6a2e9ad929a3230a22f4d64a74671a0720b. >>> General Protection Fault in rmap_walk_ksm() under memory pressure: >>> remove_rmap_item_from_tree() needs to take page lock, of course. >>> >> >> I'am really sorry about it! And many thanks for this bugfix! >> It seems rmap_walk_ksm() relies on the page lock to protect against >> concurrent modifications to that page's node of the stable tree. >> Could you please add a comment in remove_rmap_item_from_tree() to >> clarify this in case similar trouble again? Many thanks! > > Sorry, no. Page lock is held by callers of stable_tree_append() when > adding an rmap_item to the tree, and held by callers of rmap_walk_ksm() > (see VM_BUG_ON_PAGE there) when walking the tree: you would surely > expect some kind of locking when removing an rmap_item from the tree, > and the appropriate page lock is what GET_KSM_PAGE_LOCK provided. > > I do not want us to go through the kernel source adding a comment > /* We really mean to take this lock: it protects against concurrency */ > every time we take a lock in the kernel: you should generally assume > that if a lock is taken, then the writer intended it to be taken. > > There are sure to be some exceptions, where a lock is taken pointlessly: > but please look deeper before assuming that is the case. > I see. I should have been more careful. Many thanks for your detailed explanation and sorry about trouble! > Hugh > >> >>> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> >>> --- >>> >>> mm/ksm.c | 3 ++- >>> 1 file changed, 2 insertions(+), 1 deletion(-) >>> >>> --- 5.13-rc1/mm/ksm.c 2021-05-09 17:03:44.010422188 -0700 >>> +++ linux/mm/ksm.c 2021-05-09 22:12:39.403008350 -0700 >>> @@ -776,11 +776,12 @@ static void remove_rmap_item_from_tree(s >>> struct page *page; >>> >>> stable_node = rmap_item->head; >>> - page = get_ksm_page(stable_node, GET_KSM_PAGE_NOLOCK); >>> + page = get_ksm_page(stable_node, GET_KSM_PAGE_LOCK); >>> if (!page) >>> goto out; >>> >>> hlist_del(&rmap_item->hlist); >>> + unlock_page(page); >>> put_page(page); >>> >>> if (!hlist_empty(&stable_node->hlist)) > . >
