On Wed, 28 Oct 2015, Mike Kravetz wrote: > On 10/27/2015 08:34 PM, Hugh Dickins wrote: > > Thanks for the detailed response Hugh. I will try to address your questions > and provide more reasoning behind the use case and need for this code. And thank you for your detailed response, Mike: that helped a lot. > Ok, here is a bit more explanation of the proposed use case. It all > revolves around a DB's use of hugetlbfs and the desire for more control > over the underlying memory. This additional control is achieved by > adding existing fallocate and userfaultfd semantics to hugetlbfs. > > In this use case there is a single process that manages hugetlbfs files > and the underlying memory resources. It pre-allocates/initializes these > files. > > In addition, there are many other processes which access (rw mode) these > files. They will simply mmap the files. It is expected that they will > not fault in any new pages. Rather, all pages would have been pre-allocated > by the management process. > > At some time, the management process determines that specific ranges of > pages within the hugetlbfs files are no longer needed. It will then punch > holes in the files. These 'free' pages within the holes may then be used > for other purposes. For applications like this (sophisticated DBs), huge > pages are reserved at system init time and closely managed by the > application. > Hence, the desire for this additional control. > > So, when a hole containing N huge pages is punched, the management process > wants to know that it really has N huge pages for other purposes. Ideally, > none of the other processes mapping this file/area would access the hole. > This is an application error, and it can be 'caught' with userfaultfd. > > Since these other (non-management) processes will never fault in pages, > they would simply set up userfaultfd to catch any page faults immediately > after mmaping the hugetlbfs file. > > > > > But it sounds to me more as if the holes you want punched are not > > quite like on other filesystems, and you want to be able to police > > them afterwards with userfaultfd, to prevent them from being refilled. > > I am not sure if they are any different. > > One could argue that a hole punch operation must always result in all > pages within the hole being deallocated. As you point out, this could > race with a fault. Previously, there would be no way to determine if > all pages had been deallocated because user space could not detect this > race. Now, userfaultfd allows user space to catch page faults. So, > it is now possible to catch/depend on hole punch deallocating all pages > within the hole. > > > > > Can't userfaultfd be used just slightly earlier, to prevent them from > > being filled while doing the holepunch? Then no need for this patchset? > > I do not think so, at least with current userfaultfd semantics. The hole > needs to be punched before being caught with UFFDIO_REGISTER_MODE_MISSING. Great, that makes sense. I was worried that you needed some kind of atomic treatment of the whole extent punched, but all you need is to close the hole/fault race one hugepage at a time. Throw away all of 1/4, 2/4, 3/4: I think all you need is your 4/4 (plus i_mmap_lock_write around the hugetlb_vmdelete_list of course). There you already do the single hugepage hugetlb_vmdelete_list() under mutex_lock(&hugetlb_fault_mutex_table[hash]). And it should come as no surprise that hugetlb_fault() does most of its work under that same mutex. So once remove_inode_hugepages() unlocks the mutex, that page is gone from the file, and userfaultfd UFFDIO_REGISTER_MODE_MISSING will do what you want, won't it? I don't think "my" code buys you anything at all: you're not in danger of shmem's starvation livelock issue, partly because remove_inode_hugepages() uses the simple loop from start to end, and partly because hugetlb_fault() already takes the serializing mutex (no equivalent in shmem_fault()). Or am I dreaming? Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>