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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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