Re: open(2) says O_DIRECT works on 512 byte boundries?

KAMEZAWA Hiroyuki
2009-02-03T01:29:20Z

On Mon, 2 Feb 2009 23:08:56 +0100
Andrea Arcangeli <aarcange@redhat.com> wrote:

> Hi Greg!
>
> > Thanks for the pointers, I'll go read the thread and follow up there.
>
> If you also run into this final fix is attached below. Porting to
> mainline is a bit hard because of gup-fast... Perhaps we can use mmu
> notifiers to fix gup-fast... need to think more about it then I'll
> post something.
>
> Please help testing the below on pre-gup-fast kernels, thanks!
>
I commented in FJ-Redhat Path but not forwared from unknown reason ;)
I comment again.

1. Why TestSetLockPage() is necessary ?
It seems not necesary.

2. This patch doesn't cover HugeTLB.

3. Why "follow_page() successfully finds a page" case only ?
not necessary to insert SetPageGUP() in following path ?

- handle_mm_fault()
=> do_anonymos/swap/wp_page()
or some.

BTW, when you write a patch against upstream, please CC me or linux-mm.
I'll have to add a hook for memory-cgroup.

Thanks,
-Kame

> From: Andrea Arcangeli <aarcange@redhat.com>
> Subject: fork-o_direct-race
>
> Think a thread writing constantly to the last 512bytes of a page, while another
> thread read and writes to/from the first 512bytes of the page. We can lose
> O_DIRECT reads (or any other get_user_pages write=1 I/O not just bio/O_DIRECT),
> the very moment we mark any pte wrprotected because a third unrelated thread
> forks off a child.
>
> This fixes it by never wprotecting anon ptes if there can be any direct I/O in
> flight to the page, and by instantiating a readonly pte and triggering a COW in
> the child. The only trouble here are O_DIRECT reads (writes to memory, read
> from disk). Checking the page_count under the PT lock guarantees no
> get_user_pages could be running under us because if somebody wants to write to
> the page, it has to break any cow first and that requires taking the PT lock in
> follow_page before increasing the page count. We are guaranteed mapcount is 1 if
> fork is writeprotecting the pte so the PT lock is enough to serialize against
> get_user_pages->get_page.
>
> The COW triggered inside fork will run while the parent pte is readonly to
> provide as usual the per-page atomic copy from parent to child during fork.
> However timings will be altered by having to copy the pages that might be under
> O_DIRECT.
>
> The pagevec code calls get_page while the page is sitting in the pagevec
> (before it becomes PageLRU) and doing so it can generate false positives, so to
> avoid slowing down fork all the time even for pages that could never possibly
> be under O_DIRECT write=1, the PG_gup bitflag is added, this eliminates
> most overhead of the fix in fork.
>
> Patch doesn't break kABI despite introducing a new page flag.
>
> Fixed version of original patch from Nick Piggin.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
>
> diff -ur 2/include/linux/page-flags.h 1/include/linux/page-flags.h
> --- 2/include/linux/page-flags.h 2008-07-10 17:26:44.000000000 +0200
> +++ 1/include/linux/page-flags.h 2009-02-02 05:33:42.000000000 +0100
> @@ -86,6 +86,7 @@
> #define PG_reclaim 17 /* To be reclaimed asap */
> #define PG_nosave_free 18 /* Free, should not be written */
> #define PG_buddy 19 /* Page is free, on buddy lists */
> +#define PG_gup 20 /* Page pin may be because of gup */
>
> /* PG_owner_priv_1 users should have descriptive aliases */
> #define PG_checked PG_owner_priv_1 /* Used by some filesystems */
> @@ -238,6 +239,10 @@
> #define __SetPageCompound(page) __set_bit(PG_compound, &(page)->flags)
> #define __ClearPageCompound(page) __clear_bit(PG_compound, &(page)->flags)
>
> +#define SetPageGUP(page) set_bit(PG_gup, &(page)->flags)
> +#define PageGUP(page) test_bit(PG_gup, &(page)->flags)
> +#define __ClearPageGUP(page) __clear_bit(PG_gup, &(page)->flags)
> +
> /*
> * PG_reclaim is used in combination with PG_compound to mark the
> * head and tail of a compound page
> diff -ur 2/kernel/fork.c 1/kernel/fork.c
> --- 2/kernel/fork.c 2008-07-10 17:26:43.000000000 +0200
> +++ 1/kernel/fork.c 2009-02-02 05:24:17.000000000 +0100
> @@ -368,7 +368,7 @@
> rb_parent = &tmp->vm_rb;
>
> mm->map_count++;
> - retval = copy_page_range(mm, oldmm, mpnt);
> + retval = copy_page_range(mm, oldmm, tmp);
>
> if (tmp->vm_ops && tmp->vm_ops->open)
> tmp->vm_ops->open(tmp);
> Only in 1: ldtest7557.out
> diff -ur 2/mm/memory.c 1/mm/memory.c
> --- 2/mm/memory.c 2008-07-10 17:26:44.000000000 +0200
> +++ 1/mm/memory.c 2009-02-02 06:17:05.000000000 +0100
> @@ -426,7 +426,7 @@
> * covered by this vma.
> */
>
> -static inline void
> +static inline int
> copy_one_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)
> @@ -434,6 +434,7 @@
> unsigned long vm_flags = vma->vm_flags;
> pte_t pte = *src_pte;
> struct page *page;
> + int forcecow = 0;
>
> /* pte contains position in swap or file, so copy. */
> if (unlikely(!pte_present(pte))) {
> @@ -464,15 +465,6 @@
> }
>
> /*
> - * If it's a COW mapping, write protect it both
> - * in the parent and the child
> - */
> - if (is_cow_mapping(vm_flags)) {
> - ptep_set_wrprotect(src_mm, addr, src_pte);
> - pte = *src_pte;
> - }
> -
> - /*
> * If it's a shared mapping, mark it clean in
> * the child
> */
> @@ -484,13 +476,41 @@
> if (page) {
> get_page(page);
> page_dup_rmap(page);
> + if (is_cow_mapping(vm_flags) && PageAnon(page) &&
> + PageGUP(page)) {
> + if (unlikely(TestSetPageLocked(page)))
> + forcecow = 1;
> + else {
> + if (unlikely(page_count(page) !=
> + page_mapcount(page)
> + + !!PageSwapCache(page)))
> + forcecow = 1;
> + unlock_page(page);
> + }
> + }
> rss[!!PageAnon(page)]++;
> }
>
> + /*
> + * If it's a COW mapping, write protect it both
> + * in the parent and the child.
> + */
> + if (is_cow_mapping(vm_flags)) {
> + ptep_set_wrprotect(src_mm, addr, src_pte);
> + pte = pte_wrprotect(pte);
> + }
> +
> out_set_pte:
> set_pte_at(dst_mm, addr, dst_pte, pte);
> +
> + return forcecow;
> }
>
> +static void fork_pre_cow(struct mm_struct *mm, struct vm_area_struct *vma,
> + unsigned long address,
> + pte_t *src_pte, pte_t *dst_pte,
> + struct page *pre_cow_page);
> +
> static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> pmd_t *dst_pmd, pmd_t *src_pmd, struct vm_area_struct *vma,
> unsigned long addr, unsigned long end)
> @@ -499,17 +519,30 @@
> spinlock_t *src_ptl, *dst_ptl;
> int progress = 0;
> int rss[2];
> + int forcecow;
> + struct page *pre_cow_page = NULL;
>
> again:
> + if (!pre_cow_page) {
> + pre_cow_page = alloc_page_vma(GFP_HIGHUSER, vma, addr);
> + if (!pre_cow_page)
> + return -ENOMEM;
> + }
> + forcecow = 0;
> rss[1] = rss[0] = 0;
> dst_pte = pte_alloc_map_lock(dst_mm, dst_pmd, addr, &dst_ptl);
> - if (!dst_pte)
> + if (!dst_pte) {
> + page_cache_release(pre_cow_page);
> return -ENOMEM;
> + }
> src_pte = pte_offset_map_nested(src_pmd, addr);
> src_ptl = pte_lockptr(src_mm, src_pmd);
> spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
>
> do {
> + if (forcecow)
> + break;
> +
> /*
> * We are holding two locks at this point - either of them
> * could generate latencies in another task on another CPU.
> @@ -525,10 +558,26 @@
> progress++;
> continue;
> }
> - copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vma, addr, rss);
> + forcecow = copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vma, addr, rss);
> progress += 8;
> } while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
>
> + if (unlikely(forcecow)) {
> + /*
> + * Try to COW the child page as direct I/O is working
> + * on the parent page, and so we've to mark the parent
> + * pte read-write before dropping the PT lock to avoid
> + * the page to be cowed in the parent and any direct
> + * I/O to get lost.
> + */
> + fork_pre_cow(dst_mm, vma, addr-PAGE_SIZE,
> + src_pte-1, dst_pte-1, pre_cow_page);
> + /* after the page copy set the parent pte writeable */
> + set_pte_at(src_mm, addr-PAGE_SIZE, src_pte-1,
> + pte_mkwrite(*(src_pte-1)));
> + pre_cow_page = NULL;
> + }
> +
> spin_unlock(src_ptl);
> pte_unmap_nested(src_pte - 1);
> add_mm_rss(dst_mm, rss[0], rss[1]);
> @@ -536,6 +585,8 @@
> cond_resched();
> if (addr != end)
> goto again;
> + if (pre_cow_page)
> + page_cache_release(pre_cow_page);
> return 0;
> }
>
> @@ -956,8 +1007,11 @@
> if (unlikely(!page))
> goto unlock;
>
> - if (flags & FOLL_GET)
> + if (flags & FOLL_GET) {
> get_page(page);
> + if ((flags & FOLL_WRITE) && PageAnon(page) && !PageGUP(page))
> + SetPageGUP(page);
> + }
> if (flags & FOLL_TOUCH) {
> if ((flags & FOLL_WRITE) &&
> !pte_dirty(pte) && !PageDirty(page))
> @@ -1607,6 +1661,30 @@
> copy_user_highpage(dst, src, va);
> }
>
> +static void fork_pre_cow(struct mm_struct *mm, struct vm_area_struct *vma,
> + unsigned long address,
> + pte_t *src_pte, pte_t *dst_pte,
> + struct page *pre_cow_page)
> +{
> + pte_t entry;
> + struct page *old_page;
> +
> + old_page = vm_normal_page(vma, address, *src_pte);
> + BUG_ON(!old_page);
> + cow_user_page(pre_cow_page, old_page, address);
> + page_remove_rmap(old_page);
> + page_cache_release(old_page);
> +
> + flush_cache_page(vma, address, pte_pfn(*src_pte));
> + entry = mk_pte(pre_cow_page, vma->vm_page_prot);
> + entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> + page_add_new_anon_rmap(pre_cow_page, vma, address);
> + lru_cache_add_active(pre_cow_page);
> + set_pte_at(mm, address, dst_pte, entry);
> + update_mmu_cache(vma, address, entry);
> + lazy_mmu_prot_update(entry);
> +}
> +
> /*
> * This routine handles present pages, when users try to write
> * to a shared page. It is done by copying the page to a new address
> diff -ur 2/mm/page_alloc.c 1/mm/page_alloc.c
> --- 2/mm/page_alloc.c 2008-07-10 17:26:44.000000000 +0200
> +++ 1/mm/page_alloc.c 2009-02-02 05:33:18.000000000 +0100
> @@ -154,6 +154,7 @@
> 1 << PG_slab |
> 1 << PG_swapcache |
> 1 << PG_writeback |
> + 1 << PG_gup |
> 1 << PG_buddy );
> set_page_count(page, 0);
> reset_page_mapcount(page);
> @@ -400,6 +401,8 @@
> bad_page(page);
> if (PageDirty(page))
> __ClearPageDirty(page);
> + if (PageGUP(page))
> + __ClearPageGUP(page);
> /*
> * For now, we report if PG_reserved was found set, but do not
> * clear it, and do not free the page. But we shall soon need
> @@ -546,6 +549,7 @@
> 1 << PG_swapcache |
> 1 << PG_writeback |
> 1 << PG_reserved |
> + 1 << PG_gup |
> 1 << PG_buddy ))))
> bad_page(page);
>
>