| Message ID | 20231001005659.2185316-1-riel@surriel.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | hugetlbfs: close race between MADV_DONTNEED and page fault | expand |
On Sat, 30 Sep 2023 20:55:47 -0400 riel@surriel.com wrote: > v5: somehow a __vma_private_lock(vma) test failed to make it from my tree into the v4 series, fix that > v4: fix unmap_vmas locking issue pointed out by Mike Kravetz, and resulting lockdep fallout > v3: fix compile error w/ lockdep and test case errors with patch 3 > v2: fix the locking bug found with the libhugetlbfs tests. > > Malloc libraries, like jemalloc and tcalloc, take decisions on when > to call madvise independently from the code in the main application. > > This sometimes results in the application page faulting on an address, > right after the malloc library has shot down the backing memory with > MADV_DONTNEED. > > Usually this is harmless, because we always have some 4kB pages > sitting around to satisfy a page fault. However, with hugetlbfs > systems often allocate only the exact number of huge pages that > the application wants. > > Due to TLB batching, hugetlbfs MADV_DONTNEED will free pages outside of > any lock taken on the page fault path, which can open up the following > race condition: > > CPU 1 CPU 2 > > MADV_DONTNEED > unmap page > shoot down TLB entry > page fault > fail to allocate a huge page > killed with SIGBUS > free page > > Fix that race by extending the hugetlb_vma_lock locking scheme to also > cover private hugetlb mappings (with resv_map), and pulling the locking > from __unmap_hugepage_final_range into helper functions called from > zap_page_range_single. This ensures page faults stay locked out of > the MADV_DONTNEED VMA until the huge pages have actually been freed. Didn't we decide that [1/3] and [2/3] should be cc:stable? > The third patch in the series is more of an RFC. Using the > invalidate_lock instead of the hugetlb_vma_lock greatly simplifies > the code, but at the cost of turning a per-VMA lock into a lock > per backing hugetlbfs file, which could slow things down when > multiple processes are mapping the same hugetlbfs file. "could slow things down" is testable-for? This third one I'd queue up for testing for a 6.7-rc1 merge, so I'll split the series apart. Not a problem, but it would be a little better if things were originally packaged that way.
v5: somehow a __vma_private_lock(vma) test failed to make it from my tree into the v4 series, fix that v4: fix unmap_vmas locking issue pointed out by Mike Kravetz, and resulting lockdep fallout v3: fix compile error w/ lockdep and test case errors with patch 3 v2: fix the locking bug found with the libhugetlbfs tests. Malloc libraries, like jemalloc and tcalloc, take decisions on when to call madvise independently from the code in the main application. This sometimes results in the application page faulting on an address, right after the malloc library has shot down the backing memory with MADV_DONTNEED. Usually this is harmless, because we always have some 4kB pages sitting around to satisfy a page fault. However, with hugetlbfs systems often allocate only the exact number of huge pages that the application wants. Due to TLB batching, hugetlbfs MADV_DONTNEED will free pages outside of any lock taken on the page fault path, which can open up the following race condition: CPU 1 CPU 2 MADV_DONTNEED unmap page shoot down TLB entry page fault fail to allocate a huge page killed with SIGBUS free page Fix that race by extending the hugetlb_vma_lock locking scheme to also cover private hugetlb mappings (with resv_map), and pulling the locking from __unmap_hugepage_final_range into helper functions called from zap_page_range_single. This ensures page faults stay locked out of the MADV_DONTNEED VMA until the huge pages have actually been freed. The third patch in the series is more of an RFC. Using the invalidate_lock instead of the hugetlb_vma_lock greatly simplifies the code, but at the cost of turning a per-VMA lock into a lock per backing hugetlbfs file, which could slow things down when multiple processes are mapping the same hugetlbfs file.