On 3/24/21 1:43 AM, Michal Hocko wrote: > On Tue 23-03-21 11:51:04, Mike Kravetz wrote: >> On 3/22/21 11:10 AM, Roman Gushchin wrote: >>> On Mon, Mar 22, 2021 at 10:42:23AM -0700, Mike Kravetz wrote: >>>> Cc: Roman, Christoph >>>> >>>> On 3/22/21 1:41 AM, Peter Zijlstra wrote: >>>>> On Fri, Mar 19, 2021 at 03:42:08PM -0700, Mike Kravetz wrote: >>>>>> The locks acquired in free_huge_page are irq safe. However, in certain >>>>>> circumstances the routine update_and_free_page could sleep. Since >>>>>> free_huge_page can be called from any context, it can not sleep. >>>>>> >>>>>> Use a waitqueue to defer freeing of pages if the operation may sleep. A >>>>>> new routine update_and_free_page_no_sleep provides this functionality >>>>>> and is only called from free_huge_page. >>>>>> >>>>>> Note that any 'pages' sent to the workqueue for deferred freeing have >>>>>> already been removed from the hugetlb subsystem. What is actually >>>>>> deferred is returning those base pages to the low level allocator. >>>>> >>>>> So maybe I'm stupid, but why do you need that work in hugetlb? Afaict it >>>>> should be in cma_release(). >>>> >>>> My thinking (which could be totally wrong) is that cma_release makes no >>>> claims about calling context. From the code, it is pretty clear that it >>>> can only be called from task context with no locks held. Although, >>>> there could be code incorrectly calling it today hugetlb does. Since >>>> hugetlb is the only code with this new requirement, it should do the >>>> work. >>>> >>>> Wait!!! That made me remember something. >>>> Roman had code to create a non-blocking version of cma_release(). >>>> https://lore.kernel.org/linux-mm/20201022225308.2927890-1-guro@xxxxxx/ >>>> >>>> There were no objections, and Christoph even thought there may be >>>> problems with callers of dma_free_contiguous. >>>> >>>> Perhaps, we should just move forward with Roman's patches to create >>>> cma_release_nowait() and avoid this workqueue stuff? >>> >>> Sounds good to me. If it's the preferred path, I can rebase and resend >>> those patches (they been carried for some time by Zi Yan for his 1GB THP work, >>> but they are completely independent). >> >> Thanks Roman, >> >> Yes, this is the preferred path. If there is a non blocking version of >> cma_release, then it makes fixup of hugetlb put_page path much easier. > > I do not object to the plan I just want to point out that the sparse > vmemmap for hugetlb pages will need to recognize sleep/nosleep variants > of the freeing path as well to handle its vmemmap repopulate games. > Yes, I also commented elsewhere that we will likely want to do the drop/reacquire lock for each page in the looping page free routines when adding the vmemmap freeing support. Unless someone thinks otherwise, I still think it is better to first fix the hugetlb put_page/free_huge_page path with this series. Then move on to the free vmemmap series. -- Mike Kravetz
