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. Besides that I have requested to have an explanation of why blocking on the WQ is safe and that hasn't happened. -- Michal Hocko SUSE Labs
