It allows copy_pte_range() to do early cow if the pages were pinned on the source mm. Currently we don't have an accurate way to know whether a page is pinned or not. The only thing we have is page_maybe_dma_pinned(). However that's good enough for now. Especially, with the newly added mm->has_pinned flag to make sure we won't affect processes that never pinned any pages.
It would be easier if we can do GFP_KERNEL allocation within copy_one_pte(). Unluckily, we can't because we're with the page table locks held for both the parent and child processes. So the page allocation needs to be done outside copy_one_pte(). Some trick is there in copy_present_pte(), majorly the wrprotect trick to block concurrent fast-gup. Comments in the function should explain better in place. Oleg Nesterov reported a (probably harmless) bug during review that we didn't reset entry.val properly in copy_pte_range() so that potentially there's chance to call add_swap_count_continuation() multiple times on the same swp entry. However that should be harmless since even if it happens, the same function (add_swap_count_continuation()) will return directly noticing that there're enough space for the swp counter. So instead of a standalone stable patch, it is touched up in this patch directly. Reference discussions: https://lore.kernel.org/lkml/20200914143829.ga1424...@nvidia.com/ Suggested-by: Linus Torvalds <torva...@linux-foundation.org> Signed-off-by: Peter Xu <pet...@redhat.com> --- mm/memory.c | 172 +++++++++++++++++++++++++++++++++++++++++++++++----- 1 file changed, 156 insertions(+), 16 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 4c56d7b92b0e..92ad08616e60 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -773,15 +773,109 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, return 0; } -static inline void +/* + * Copy one pte. Returns 0 if succeeded, or -EAGAIN if one preallocated page + * is required to copy this pte. + */ +static inline int copy_present_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, pte_t *dst_pte, pte_t *src_pte, struct vm_area_struct *vma, - unsigned long addr, int *rss) + struct vm_area_struct *new, + unsigned long addr, int *rss, struct page **prealloc) { unsigned long vm_flags = vma->vm_flags; pte_t pte = *src_pte; struct page *page; + page = vm_normal_page(vma, addr, pte); + if (page) { + if (is_cow_mapping(vm_flags)) { + bool is_write = pte_write(pte); + + /* + * The trick starts. + * + * What we want to do is to check whether this page may + * have been pinned by the parent process. If so, + * instead of wrprotect the pte on both sides, we copy + * the page immediately so that we'll always guarantee + * the pinned page won't be randomly replaced in the + * future. + * + * To achieve this, we do the following: + * + * 1. Write-protect the pte if it's writable. This is + * to protect concurrent write fast-gup with + * FOLL_PIN, so that we'll fail the fast-gup with + * the write bit removed. + * + * 2. Check page_maybe_dma_pinned() to see whether this + * page may have been pinned. + * + * The order of these steps is important to serialize + * against the fast-gup code (gup_pte_range()) on the + * pte check and try_grab_compound_head(), so that + * we'll make sure either we'll capture that fast-gup + * so we'll copy the pinned page here, or we'll fail + * that fast-gup. + */ + if (is_write) { + ptep_set_wrprotect(src_mm, addr, src_pte); + /* + * This is not needed for serializing fast-gup, + * however always make it consistent with + * src_pte, since we'll need it when current + * page is not pinned. + */ + pte = pte_wrprotect(pte); + } + + if (atomic_read(&src_mm->has_pinned) && + page_maybe_dma_pinned(page)) { + struct page *new_page = *prealloc; + + /* + * This is possibly pinned page, need to copy. + * Safe to release the write bit if necessary. + */ + if (is_write) + set_pte_at(src_mm, addr, src_pte, + pte_mkwrite(pte)); + + /* If we don't have a pre-allocated page, ask */ + if (!new_page) + return -EAGAIN; + + /* + * We have a prealloc page, all good! Take it + * over and copy the page & arm it. + */ + *prealloc = NULL; + copy_user_highpage(new_page, page, addr, vma); + __SetPageUptodate(new_page); + pte = mk_pte(new_page, new->vm_page_prot); + pte = pte_sw_mkyoung(pte); + pte = maybe_mkwrite(pte_mkdirty(pte), new); + page_add_new_anon_rmap(new_page, new, addr, false); + rss[mm_counter(new_page)]++; + set_pte_at(dst_mm, addr, dst_pte, pte); + return 0; + } + + /* + * Logically we should recover the wrprotect() for + * fast-gup, however when reach here it also means we + * actually need to wrprotect() it again for cow. + * Simply keep everything. Note that there's another + * chunk of cow logic below, but we should still need + * that for !page case. + */ + } + get_page(page); + page_dup_rmap(page, false); + rss[mm_counter(page)]++; + } + /* * If it's a COW mapping, write protect it both * in the parent and the child @@ -807,14 +901,27 @@ copy_present_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm, if (!(vm_flags & VM_UFFD_WP)) pte = pte_clear_uffd_wp(pte); - page = vm_normal_page(vma, addr, pte); - if (page) { - get_page(page); - page_dup_rmap(page, false); - rss[mm_counter(page)]++; + set_pte_at(dst_mm, addr, dst_pte, pte); + return 0; +} + +static inline struct page * +page_copy_prealloc(struct mm_struct *src_mm, struct vm_area_struct *vma, + unsigned long addr) +{ + struct page *new_page; + + new_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, addr); + if (!new_page) + return NULL; + + if (mem_cgroup_charge(new_page, src_mm, GFP_KERNEL)) { + put_page(new_page); + return NULL; } + cgroup_throttle_swaprate(new_page, GFP_KERNEL); - set_pte_at(dst_mm, addr, dst_pte, pte); + return new_page; } static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, @@ -825,16 +932,20 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, pte_t *orig_src_pte, *orig_dst_pte; pte_t *src_pte, *dst_pte; spinlock_t *src_ptl, *dst_ptl; - int progress = 0; + int progress, ret = 0; int rss[NR_MM_COUNTERS]; swp_entry_t entry = (swp_entry_t){0}; + struct page *prealloc = NULL; again: + progress = 0; init_rss_vec(rss); dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl); - if (!dst_pte) - return -ENOMEM; + if (!dst_pte) { + ret = -ENOMEM; + goto out; + } src_pte = pte_offset_map(src_pmd, addr); src_ptl = pte_lockptr(src_mm, src_pmd); spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); @@ -866,8 +977,25 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, progress += 8; continue; } - copy_present_pte(dst_mm, src_mm, dst_pte, src_pte, - vma, addr, rss); + /* copy_present_pte() will clear `*prealloc' if consumed */ + ret = copy_present_pte(dst_mm, src_mm, dst_pte, src_pte, + vma, new, addr, rss, &prealloc); + /* + * If we need a pre-allocated page for this pte, drop the + * locks, allocate, and try again. + */ + if (unlikely(ret == -EAGAIN)) + break; + if (unlikely(prealloc)) { + /* + * pre-alloc page cannot be reused by next time so as + * to strictly follow mempolicy (e.g., alloc_page_vma() + * will allocate page according to address). This + * could only happen if one pinned pte changed. + */ + put_page(prealloc); + prealloc = NULL; + } progress += 8; } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end); @@ -879,13 +1007,25 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, cond_resched(); if (entry.val) { - if (add_swap_count_continuation(entry, GFP_KERNEL) < 0) + if (add_swap_count_continuation(entry, GFP_KERNEL) < 0) { + ret = -ENOMEM; + goto out; + } + entry.val = 0; + } else if (ret) { + WARN_ON_ONCE(ret != -EAGAIN); + prealloc = page_copy_prealloc(src_mm, vma, addr); + if (!prealloc) return -ENOMEM; - progress = 0; + /* We've captured and resolved the error. Reset, try again. */ + ret = 0; } if (addr != end) goto again; - return 0; +out: + if (unlikely(prealloc)) + put_page(prealloc); + return ret; } static inline int copy_pmd_range(struct mm_struct *dst_mm, struct mm_struct *src_mm, -- 2.26.2
