On Fri, Jan 8, 2021 at 8:04 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Fri 08-01-21 19:52:54, Muchun Song wrote: > > On Fri, Jan 8, 2021 at 7:44 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Fri 08-01-21 18:08:57, Muchun Song wrote: > > > > On Fri, Jan 8, 2021 at 5:31 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > On Fri 08-01-21 17:01:03, Muchun Song wrote: > > > > > > On Fri, Jan 8, 2021 at 4:43 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > > > > > On Thu 07-01-21 23:11:22, Muchun Song wrote: > > > > > [..] > > > > > > > > But I find a tricky problem to solve. See free_huge_page(). > > > > > > > > If we are in non-task context, we should schedule a work > > > > > > > > to free the page. We reuse the page->mapping. If the page > > > > > > > > is already freed by the dissolve path. We should not touch > > > > > > > > the page->mapping. So we need to check PageHuge(). > > > > > > > > The check and llist_add() should be protected by > > > > > > > > hugetlb_lock. But we cannot do that. Right? If dissolve > > > > > > > > happens after it is linked to the list. We also should > > > > > > > > remove it from the list (hpage_freelist). It seems to make > > > > > > > > the thing more complex. > > > > > > > > > > > > > > I am not sure I follow you here but yes PageHuge under hugetlb_lock > > > > > > > should be the reliable way to check for the race. I am not sure why we > > > > > > > really need to care about mapping or other state. > > > > > > > > > > > > CPU0: CPU1: > > > > > > free_huge_page(page) > > > > > > if (PageHuge(page)) > > > > > > dissolve_free_huge_page(page) > > > > > > spin_lock(&hugetlb_lock) > > > > > > update_and_free_page(page) > > > > > > spin_unlock(&hugetlb_lock) > > > > > > llist_add(page->mapping) > > > > > > // the mapping is corrupted > > > > > > > > > > > > The PageHuge(page) and llist_add() should be protected by > > > > > > hugetlb_lock. Right? If so, we cannot hold hugetlb_lock > > > > > > in free_huge_page() path. > > > > > > > > > > OK, I see. I completely forgot about this snowflake. I thought that > > > > > free_huge_page was a typo missing initial __. Anyway you are right that > > > > > this path needs a check as well. But I don't see why we couldn't use the > > > > > lock here. The lock can be held only inside the !in_task branch. > > > > > > > > Because we hold the hugetlb_lock without disable irq. So if an interrupt > > > > occurs after we hold the lock. And we also free a HugeTLB page. Then > > > > it leads to deadlock. > > > > > > There is nothing really to prevent making hugetlb_lock irq safe, isn't > > > it? > > > > Yeah. We can make the hugetlb_lock irq safe. But why have we not > > done this? Maybe the commit changelog can provide more information. > > > > See https://github.com/torvalds/linux/commit/c77c0a8ac4c522638a8242fcb9de9496e3cdbb2d > > Dang! Maybe it is the time to finally stack one workaround on top of the > other and put this code into the shape. The amount of hackery and subtle > details has just grown beyond healthy! >From readability point of view, maybe making the hugetlb_lock irq safe is an improvement (in the feature). The details are always messy. :) > -- > Michal Hocko > SUSE Labs
