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    SubjectRe: [GIT PULL] KVM updates for the 2.6.38 merge window
    On Wed, Jan 12, 2011 at 12:53:14PM -0800, Linus Torvalds wrote:
    > On Wed, Jan 12, 2011 at 12:33 PM, Rik van Riel <riel@redhat.com> wrote:
    > >
    > > Now that we have FAULT_FLAG_ALLOW_RETRY, the async
    > > pagefault patches can be a little smaller.
    >
    > I suspect you do still want a new page flag, to say that
    > FAULT_FLAG_ALLOW_RETRY shouldn't actually wait for the page that it
    > allows retry for.
    >
    > But even then, that flag should not be named "MINOR", it should be
    > about what the behaviour is actually all about ("NOWAIT_RETRY" or
    > whatever - it presumably would also cause us to not drop the
    > mmap_sem).
    >
    > IOW, these days I suspect the patch _should_ look something like the attached.
    >
    > Anyway, with this, you should be able to use
    >
    > FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT
    >
    > to basically get a non-waiting page fault (and it will return the
    > VM_FAULT_RETRY error code if it failed).
    >
    I implemented get_user_pages_nowait() on top of your patch. In my testing
    it works as expected when used inside KVM. Does this looks OK to you?

    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index dc83565..d78e9e7 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -859,6 +859,9 @@ extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *
    int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    unsigned long start, int nr_pages, int write, int force,
    struct page **pages, struct vm_area_struct **vmas);
    +int get_user_pages_nowait(struct task_struct *tsk, struct mm_struct *mm,
    + unsigned long start, int nr_pages, int write, int force,
    + struct page **pages, struct vm_area_struct **vmas);
    int get_user_pages_fast(unsigned long start, int nr_pages, int write,
    struct page **pages);
    struct page *get_dump_page(unsigned long addr);
    @@ -1416,6 +1419,8 @@ struct page *follow_page(struct vm_area_struct *, unsigned long address,
    #define FOLL_GET 0x04 /* do get_page on page */
    #define FOLL_DUMP 0x08 /* give error on hole if it would be zero */
    #define FOLL_FORCE 0x10 /* get_user_pages read/write w/o permission */
    +#define FOLL_RETRY 0x20 /* if disk transfer is needed release mmap_sem and return error */
    +#define FOLL_NOWAIT 0x40 /* if disk transfer is needed return error without releasing mmap_sem */

    typedef int (*pte_fn_t)(pte_t *pte, pgtable_t token, unsigned long addr,
    void *data);
    diff --git a/mm/memory.c b/mm/memory.c
    index 02e48aa..0a3d3b5 100644
    --- a/mm/memory.c
    +++ b/mm/memory.c
    @@ -1443,8 +1443,12 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    int ret;

    ret = handle_mm_fault(mm, vma, start,
    - (foll_flags & FOLL_WRITE) ?
    - FAULT_FLAG_WRITE : 0);
    + ((foll_flags & FOLL_WRITE) ?
    + FAULT_FLAG_WRITE : 0) |
    + ((foll_flags & FOLL_RETRY) ?
    + FAULT_FLAG_ALLOW_RETRY : 0) |
    + ((foll_flags & FOLL_NOWAIT) ?
    + FAULT_FLAG_RETRY_NOWAIT : 0));

    if (ret & VM_FAULT_ERROR) {
    if (ret & VM_FAULT_OOM)
    @@ -1460,6 +1464,9 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    else
    tsk->min_flt++;

    + if (ret & VM_FAULT_RETRY)
    + return i ? i : -EFAULT;
    +
    /*
    * The VM_FAULT_WRITE bit tells us that
    * do_wp_page has broken COW when necessary,
    @@ -1563,6 +1570,23 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
    }
    EXPORT_SYMBOL(get_user_pages);

    +int get_user_pages_nowait(struct task_struct *tsk, struct mm_struct *mm,
    + unsigned long start, int nr_pages, int write, int force,
    + struct page **pages, struct vm_area_struct **vmas)
    +{
    + int flags = FOLL_TOUCH | FOLL_RETRY | FOLL_NOWAIT;
    +
    + if (pages)
    + flags |= FOLL_GET;
    + if (write)
    + flags |= FOLL_WRITE;
    + if (force)
    + flags |= FOLL_FORCE;
    +
    + return __get_user_pages(tsk, mm, start, nr_pages, flags, pages, vmas);
    +}
    +EXPORT_SYMBOL(get_user_pages_nowait);
    +
    /**
    * get_dump_page() - pin user page in memory while writing it to core dump
    * @addr: user address
    --
    Gleb.

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