From 766b59e876505852e1571142bd57dfe20fca383e Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 8 Jun 2023 12:10:57 -0700 Subject: arm: allow pte_offset_map[_lock]() to fail Patch series "arch: allow pte_offset_map[_lock]() to fail", v2. What is it all about? Some mmap_lock avoidance i.e. latency reduction. Initially just for the case of collapsing shmem or file pages to THPs; but likely to be relied upon later in other contexts e.g. freeing of empty page tables (but that's not work I'm doing). mmap_write_lock avoidance when collapsing to anon THPs? Perhaps, but again that's not work I've done: a quick attempt was not as easy as the shmem/file case. I would much prefer not to have to make these small but wide-ranging changes for such a niche case; but failed to find another way, and have heard that shmem MADV_COLLAPSE's usefulness is being limited by that mmap_write_lock it currently requires. These changes (though of course not these exact patches, and not all of these architectures!) have been in Google's data centre kernel for three years now: we do rely upon them. What are the per-arch changes about? Generally, two things. One: the current mmap locking may not be enough to guard against that tricky transition between pmd entry pointing to page table, and empty pmd entry, and pmd entry pointing to huge page: pte_offset_map() will have to validate the pmd entry for itself, returning NULL if no page table is there. What to do about that varies: often the nearby error handling indicates just to skip it; but in some cases a "goto again" looks appropriate (and if that risks an infinite loop, then there must have been an oops, or pfn 0 mistaken for page table, before). Deeper study of each site might show that 90% of them here in arch code could only fail if there's corruption e.g. a transition to THP would be surprising on an arch without HAVE_ARCH_TRANSPARENT_HUGEPAGE. But given the likely extension to freeing empty page tables, I have not limited this set of changes to THP; and it has been easier, and sets a better example, if each site is given appropriate handling. Two: pte_offset_map() will need to do an rcu_read_lock(), with the corresponding rcu_read_unlock() in pte_unmap(). But most architectures never supported CONFIG_HIGHPTE, so some don't always call pte_unmap() after pte_offset_map(), or have used userspace pte_offset_map() where pte_offset_kernel() is more correct. No problem in the current tree, but a problem once an rcu_read_unlock() will be needed to keep balance. A common special case of that comes in arch/*/mm/hugetlbpage.c, if the architecture supports hugetlb pages down at the lowest PTE level. huge_pte_alloc() uses pte_alloc_map(), but generic hugetlb code does no corresponding pte_unmap(); similarly for huge_pte_offset(). In rare transient cases, not yet made possible, pte_offset_map() and pte_offset_map_lock() may not find a page table: handle appropriately. Link: https://lkml.kernel.org/r/a4963be9-7aa6-350-66d0-2ba843e1af44@google.com Link: https://lkml.kernel.org/r/813429a1-204a-1844-eeae-7fd72826c28@google.com Signed-off-by: Hugh Dickins Cc: Alexander Gordeev Cc: Alexandre Ghiti Cc: Aneesh Kumar K.V Cc: Catalin Marinas Cc: Christian Borntraeger Cc: Chris Zankel Cc: Claudio Imbrenda Cc: David Hildenbrand Cc: "David S. Miller" Cc: Geert Uytterhoeven Cc: Greg Ungerer Cc: Heiko Carstens Cc: Helge Deller Cc: John David Anglin Cc: John Paul Adrian Glaubitz Cc: Kirill A. Shutemov Cc: Matthew Wilcox (Oracle) Cc: Max Filippov Cc: Michael Ellerman Cc: Michal Simek Cc: Mike Kravetz Cc: Mike Rapoport (IBM) Cc: Palmer Dabbelt Cc: Peter Zijlstra Cc: Qi Zheng Cc: Russell King Cc: Suren Baghdasaryan Cc: Thomas Bogendoerfer Cc: Will Deacon Cc: Ingo Molnar Cc: "H. Peter Anvin" Cc: Thomas Gleixner Signed-off-by: Andrew Morton --- arch/arm/lib/uaccess_with_memcpy.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'arch/arm/lib') diff --git a/arch/arm/lib/uaccess_with_memcpy.c b/arch/arm/lib/uaccess_with_memcpy.c index e4c2677cc1e9..2f6163f05e93 100644 --- a/arch/arm/lib/uaccess_with_memcpy.c +++ b/arch/arm/lib/uaccess_with_memcpy.c @@ -74,6 +74,9 @@ pin_page_for_write(const void __user *_addr, pte_t **ptep, spinlock_t **ptlp) return 0; pte = pte_offset_map_lock(current->mm, pmd, addr, &ptl); + if (unlikely(!pte)) + return 0; + if (unlikely(!pte_present(*pte) || !pte_young(*pte) || !pte_write(*pte) || !pte_dirty(*pte))) { pte_unmap_unlock(pte, ptl); -- cgit v1.2.3