From 26e1a0c3277d7f43856ec424902423be212cc178 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 8 Jun 2023 18:06:53 -0700 Subject: mm: use pmdp_get_lockless() without surplus barrier() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Patch series "mm: 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) have been in Google's data centre kernel for three years now: we do rely upon them. What is this preparatory series about? The current mmap locking will 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: sometimes nearby error handling indicates just to skip it; but in many cases an ACTION_AGAIN or "goto again" is appropriate (and if that risks an infinite loop, then there must have been an oops, or pfn 0 mistaken for page table, before). Given the likely extension to freeing empty page tables, I have not limited this set of changes to a THP config; and it has been easier, and sets a better example, if each site is given appropriate handling: even where deeper study might prove that failure could only happen if the pmd table were corrupted. Several of the patches are, or include, cleanup on the way; and by the end, pmd_trans_unstable() and suchlike are deleted: pte_offset_map() and pte_offset_map_lock() then handle those original races and more. Most uses of pte_lockptr() are deprecated, with pte_offset_map_nolock() taking its place. This patch (of 32): Use pmdp_get_lockless() in preference to READ_ONCE(*pmdp), to get a more reliable result with PAE (or READ_ONCE as before without PAE); and remove the unnecessary extra barrier()s which got left behind in its callers. HOWEVER: Note the small print in linux/pgtable.h, where it was designed specifically for fast GUP, and depends on interrupts being disabled for its full guarantee: most callers which have been added (here and before) do NOT have interrupts disabled, so there is still some need for caution. Link: https://lkml.kernel.org/r/f35279a9-9ac0-de22-d245-591afbfb4dc@google.com Signed-off-by: Hugh Dickins Acked-by: Yu Zhao Acked-by: Peter Xu Cc: Alistair Popple Cc: Anshuman Khandual Cc: Axel Rasmussen Cc: Christophe Leroy Cc: Christoph Hellwig Cc: David Hildenbrand Cc: "Huang, Ying" Cc: Ira Weiny Cc: Jason Gunthorpe Cc: Kirill A. Shutemov Cc: Lorenzo Stoakes Cc: Matthew Wilcox Cc: Mel Gorman Cc: Miaohe Lin Cc: Mike Kravetz Cc: Mike Rapoport (IBM) Cc: Minchan Kim Cc: Naoya Horiguchi Cc: Pavel Tatashin Cc: Peter Zijlstra Cc: Qi Zheng Cc: Ralph Campbell Cc: Ryan Roberts Cc: SeongJae Park Cc: Song Liu Cc: Steven Price Cc: Suren Baghdasaryan Cc: Thomas Hellström Cc: Will Deacon Cc: Yang Shi Cc: Zack Rusin Signed-off-by: Andrew Morton --- mm/mprotect.c | 5 ----- 1 file changed, 5 deletions(-) (limited to 'mm/mprotect.c') diff --git a/mm/mprotect.c b/mm/mprotect.c index 92d3d3ca390a..c5a13c0f1017 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -309,11 +309,6 @@ static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd) { pmd_t pmdval = pmdp_get_lockless(pmd); - /* See pmd_none_or_trans_huge_or_clear_bad for info on barrier */ -#ifdef CONFIG_TRANSPARENT_HUGEPAGE - barrier(); -#endif - if (pmd_none(pmdval)) return 1; if (pmd_trans_huge(pmdval)) -- cgit v1.2.3 From 670ddd8cdcbd1d07a4571266ae3517f821728c3a Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 8 Jun 2023 18:30:48 -0700 Subject: mm/mprotect: delete pmd_none_or_clear_bad_unless_trans_huge() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit change_pmd_range() had special pmd_none_or_clear_bad_unless_trans_huge(), required to avoid "bad" choices when setting automatic NUMA hinting under mmap_read_lock(); but most of that is already covered in pte_offset_map() now. change_pmd_range() just wants a pmd_none() check before wasting time on MMU notifiers, then checks on the read-once _pmd value to work out what's needed for huge cases. If change_pte_range() returns -EAGAIN to retry if pte_offset_map_lock() fails, nothing more special is needed. Link: https://lkml.kernel.org/r/725a42a9-91e9-c868-925-e3a5fd40bb4f@google.com Signed-off-by: Hugh Dickins Cc: Alistair Popple Cc: Anshuman Khandual Cc: Axel Rasmussen Cc: Christophe Leroy Cc: Christoph Hellwig Cc: David Hildenbrand Cc: "Huang, Ying" Cc: Ira Weiny Cc: Jason Gunthorpe Cc: Kirill A. Shutemov Cc: Lorenzo Stoakes Cc: Matthew Wilcox Cc: Mel Gorman Cc: Miaohe Lin Cc: Mike Kravetz Cc: Mike Rapoport (IBM) Cc: Minchan Kim Cc: Naoya Horiguchi Cc: Pavel Tatashin Cc: Peter Xu Cc: Peter Zijlstra Cc: Qi Zheng Cc: Ralph Campbell Cc: Ryan Roberts Cc: SeongJae Park Cc: Song Liu Cc: Steven Price Cc: Suren Baghdasaryan Cc: Thomas Hellström Cc: Will Deacon Cc: Yang Shi Cc: Yu Zhao Cc: Zack Rusin Signed-off-by: Andrew Morton --- mm/mprotect.c | 74 ++++++++++++++--------------------------------------------- 1 file changed, 17 insertions(+), 57 deletions(-) (limited to 'mm/mprotect.c') diff --git a/mm/mprotect.c b/mm/mprotect.c index c5a13c0f1017..64e1df0af514 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -93,22 +93,9 @@ static long change_pte_range(struct mmu_gather *tlb, bool uffd_wp_resolve = cp_flags & MM_CP_UFFD_WP_RESOLVE; tlb_change_page_size(tlb, PAGE_SIZE); - - /* - * Can be called with only the mmap_lock for reading by - * prot_numa so we must check the pmd isn't constantly - * changing from under us from pmd_none to pmd_trans_huge - * and/or the other way around. - */ - if (pmd_trans_unstable(pmd)) - return 0; - - /* - * The pmd points to a regular pte so the pmd can't change - * from under us even if the mmap_lock is only hold for - * reading. - */ pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl); + if (!pte) + return -EAGAIN; /* Get target node for single threaded private VMAs */ if (prot_numa && !(vma->vm_flags & VM_SHARED) && @@ -301,26 +288,6 @@ static long change_pte_range(struct mmu_gather *tlb, return pages; } -/* - * Used when setting automatic NUMA hinting protection where it is - * critical that a numa hinting PMD is not confused with a bad PMD. - */ -static inline int pmd_none_or_clear_bad_unless_trans_huge(pmd_t *pmd) -{ - pmd_t pmdval = pmdp_get_lockless(pmd); - - if (pmd_none(pmdval)) - return 1; - if (pmd_trans_huge(pmdval)) - return 0; - if (unlikely(pmd_bad(pmdval))) { - pmd_clear_bad(pmd); - return 1; - } - - return 0; -} - /* * Return true if we want to split THPs into PTE mappings in change * protection procedure, false otherwise. @@ -398,7 +365,8 @@ static inline long change_pmd_range(struct mmu_gather *tlb, pmd = pmd_offset(pud, addr); do { long ret; - + pmd_t _pmd; +again: next = pmd_addr_end(addr, end); ret = change_pmd_prepare(vma, pmd, cp_flags); @@ -406,16 +374,8 @@ static inline long change_pmd_range(struct mmu_gather *tlb, pages = ret; break; } - /* - * Automatic NUMA balancing walks the tables with mmap_lock - * held for read. It's possible a parallel update to occur - * between pmd_trans_huge() and a pmd_none_or_clear_bad() - * check leading to a false positive and clearing. - * Hence, it's necessary to atomically read the PMD value - * for all the checks. - */ - if (!is_swap_pmd(*pmd) && !pmd_devmap(*pmd) && - pmd_none_or_clear_bad_unless_trans_huge(pmd)) + + if (pmd_none(*pmd)) goto next; /* invoke the mmu notifier if the pmd is populated */ @@ -426,7 +386,8 @@ static inline long change_pmd_range(struct mmu_gather *tlb, mmu_notifier_invalidate_range_start(&range); } - if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) { + _pmd = pmdp_get_lockless(pmd); + if (is_swap_pmd(_pmd) || pmd_trans_huge(_pmd) || pmd_devmap(_pmd)) { if ((next - addr != HPAGE_PMD_SIZE) || pgtable_split_needed(vma, cp_flags)) { __split_huge_pmd(vma, pmd, addr, false, NULL); @@ -441,15 +402,10 @@ static inline long change_pmd_range(struct mmu_gather *tlb, break; } } else { - /* - * change_huge_pmd() does not defer TLB flushes, - * so no need to propagate the tlb argument. - */ - int nr_ptes = change_huge_pmd(tlb, vma, pmd, + ret = change_huge_pmd(tlb, vma, pmd, addr, newprot, cp_flags); - - if (nr_ptes) { - if (nr_ptes == HPAGE_PMD_NR) { + if (ret) { + if (ret == HPAGE_PMD_NR) { pages += HPAGE_PMD_NR; nr_huge_updates++; } @@ -460,8 +416,12 @@ static inline long change_pmd_range(struct mmu_gather *tlb, } /* fall through, the trans huge pmd just split */ } - pages += change_pte_range(tlb, vma, pmd, addr, next, - newprot, cp_flags); + + ret = change_pte_range(tlb, vma, pmd, addr, next, newprot, + cp_flags); + if (ret < 0) + goto again; + pages += ret; next: cond_resched(); } while (pmd++, addr = next, addr != end); -- cgit v1.2.3 From c33c794828f21217f72ce6fc140e0d34e0d56bff Mon Sep 17 00:00:00 2001 From: Ryan Roberts Date: Mon, 12 Jun 2023 16:15:45 +0100 Subject: mm: ptep_get() conversion MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Convert all instances of direct pte_t* dereferencing to instead use ptep_get() helper. This means that by default, the accesses change from a C dereference to a READ_ONCE(). This is technically the correct thing to do since where pgtables are modified by HW (for access/dirty) they are volatile and therefore we should always ensure READ_ONCE() semantics. But more importantly, by always using the helper, it can be overridden by the architecture to fully encapsulate the contents of the pte. Arch code is deliberately not converted, as the arch code knows best. It is intended that arch code (arm64) will override the default with its own implementation that can (e.g.) hide certain bits from the core code, or determine young/dirty status by mixing in state from another source. Conversion was done using Coccinelle: ---- // $ make coccicheck \ // COCCI=ptepget.cocci \ // SPFLAGS="--include-headers" \ // MODE=patch virtual patch @ depends on patch @ pte_t *v; @@ - *v + ptep_get(v) ---- Then reviewed and hand-edited to avoid multiple unnecessary calls to ptep_get(), instead opting to store the result of a single call in a variable, where it is correct to do so. This aims to negate any cost of READ_ONCE() and will benefit arch-overrides that may be more complex. Included is a fix for an issue in an earlier version of this patch that was pointed out by kernel test robot. The issue arose because config MMU=n elides definition of the ptep helper functions, including ptep_get(). HUGETLB_PAGE=n configs still define a simple huge_ptep_clear_flush() for linking purposes, which dereferences the ptep. So when both configs are disabled, this caused a build error because ptep_get() is not defined. Fix by continuing to do a direct dereference when MMU=n. This is safe because for this config the arch code cannot be trying to virtualize the ptes because none of the ptep helpers are defined. Link: https://lkml.kernel.org/r/20230612151545.3317766-4-ryan.roberts@arm.com Reported-by: kernel test robot Link: https://lore.kernel.org/oe-kbuild-all/202305120142.yXsNEo6H-lkp@intel.com/ Signed-off-by: Ryan Roberts Cc: Adrian Hunter Cc: Alexander Potapenko Cc: Alexander Shishkin Cc: Alex Williamson Cc: Al Viro Cc: Andrey Konovalov Cc: Andrey Ryabinin Cc: Christian Brauner Cc: Christoph Hellwig Cc: Daniel Vetter Cc: Dave Airlie Cc: Dimitri Sivanich Cc: Dmitry Vyukov Cc: Ian Rogers Cc: Jason Gunthorpe Cc: Jérôme Glisse Cc: Jiri Olsa Cc: Johannes Weiner Cc: Kirill A. Shutemov Cc: Lorenzo Stoakes Cc: Mark Rutland Cc: Matthew Wilcox Cc: Miaohe Lin Cc: Michal Hocko Cc: Mike Kravetz Cc: Mike Rapoport (IBM) Cc: Muchun Song Cc: Namhyung Kim Cc: Naoya Horiguchi Cc: Oleksandr Tyshchenko Cc: Pavel Tatashin Cc: Roman Gushchin Cc: SeongJae Park Cc: Shakeel Butt Cc: Uladzislau Rezki (Sony) Cc: Vincenzo Frascino Cc: Yu Zhao Signed-off-by: Andrew Morton --- mm/mprotect.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) (limited to 'mm/mprotect.c') diff --git a/mm/mprotect.c b/mm/mprotect.c index 64e1df0af514..327a6eb90afb 100644 --- a/mm/mprotect.c +++ b/mm/mprotect.c @@ -105,7 +105,7 @@ static long change_pte_range(struct mmu_gather *tlb, flush_tlb_batched_pending(vma->vm_mm); arch_enter_lazy_mmu_mode(); do { - oldpte = *pte; + oldpte = ptep_get(pte); if (pte_present(oldpte)) { pte_t ptent; @@ -544,7 +544,8 @@ long change_protection(struct mmu_gather *tlb, static int prot_none_pte_entry(pte_t *pte, unsigned long addr, unsigned long next, struct mm_walk *walk) { - return pfn_modify_allowed(pte_pfn(*pte), *(pgprot_t *)(walk->private)) ? + return pfn_modify_allowed(pte_pfn(ptep_get(pte)), + *(pgprot_t *)(walk->private)) ? 0 : -EACCES; } @@ -552,7 +553,8 @@ static int prot_none_hugetlb_entry(pte_t *pte, unsigned long hmask, unsigned long addr, unsigned long next, struct mm_walk *walk) { - return pfn_modify_allowed(pte_pfn(*pte), *(pgprot_t *)(walk->private)) ? + return pfn_modify_allowed(pte_pfn(ptep_get(pte)), + *(pgprot_t *)(walk->private)) ? 0 : -EACCES; } -- cgit v1.2.3