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. -- Michal Hocko SUSE Labs
