On Tue, Jan 12, 2021 at 4:33 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Mon 11-01-21 17:20:51, Mike Kravetz wrote: > > On 1/10/21 4:40 AM, Muchun Song wrote: > > > There is a race between dissolve_free_huge_page() and put_page(), > > > and the race window is quite small. Theoretically, we should return > > > -EBUSY when we encounter this race. In fact, we have a chance to > > > successfully dissolve the page if we do a retry. Because the race > > > window is quite small. If we seize this opportunity, it is an > > > optimization for increasing the success rate of dissolving page. > > > > > > If we free a HugeTLB page from a non-task context, it is deferred > > > through a workqueue. In this case, we need to flush the work. > > > > > > The dissolve_free_huge_page() can be called from memory hotplug, > > > the caller aims to free the HugeTLB page to the buddy allocator > > > so that the caller can unplug the page successfully. > > > > > > Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> > > > --- > > > mm/hugetlb.c | 26 +++++++++++++++++++++----- > > > 1 file changed, 21 insertions(+), 5 deletions(-) > > > > I am unsure about the need for this patch. The code is OK, there are no > > issues with the code. > > > > As mentioned in the commit message, this is an optimization and could > > potentially cause a memory offline operation to succeed instead of fail. > > However, we are very unlikely to ever exercise this code. Adding an > > optimization that is unlikely to be exercised is certainly questionable. > > > > Memory offline is the only code that could benefit from this optimization. > > As someone with more memory offline user experience, what is your opinion > > Michal? > > I am not a great fun of optimizations without any data to back them up. > I do not see any sign this code has been actually tested and the > condition triggered. This race is quite small. I only trigger this only once on my server. And then the kernel panic. So I sent this patch series to fix some bugs. > > Besides that I have requested to have an explanation of why blocking on > the WQ is safe and that hasn't happened. I have seen all the caller of dissolve_free_huge_page, some caller is under page lock (via lock_page). Others are also under a sleep context. So I think that blocking on the WQ is safe. Right? > -- > Michal Hocko > SUSE Labs
