On 10/28/2015 02:00 PM, Hugh Dickins wrote: > 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? I don't think you are dreaming. I should have stepped back and thought about this more before before pulling in the shmem code. It really is only a 'page at a time' operation, and we can use the fault mutex table for that. I'll code it up with just the changes needed for 4/4 and put it through some stress testing. Thanks, -- Mike Kravetz > > 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>