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SubjectRe: [PATCH 02/37] mm, frame_vector: do not use get_user_pages_locked()
On Mon 16-11-15 19:35:14, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> get_user_pages_locked() appears to be for use when a caller needs
> to know that its lock on mmap_sem was invalidated by the gup
> call.
>
> But, get_vaddr_frames() is not one of those users. It
> unconditionally locks the mmap_sem and unconditionally unlocks it
> after the gup call. It takes no special action and does not need
> to know whether its lock was invalidated or not.
>
> Replace get_user_pages_locked() with a vanilla get_user_pages()
> and save a few lines of code.
>
> Note that this was the *ONLY* use of get_user_pages_locked() in
> the entire kernel tree.

I've used get_user_pages_locked() because of a comment before that function
saying:

* We can leverage the VM_FAULT_RETRY functionality in the page fault
* paths better by using either get_user_pages_locked() or
* get_user_pages_unlocked().
*
* get_user_pages_locked() is suitable to replace the form:
*
* down_read(&mm->mmap_sem);
* do_something()
* get_user_pages(tsk, mm, ..., pages, NULL);
* up_read(&mm->mmap_sem);
*
* to:
*
* int locked = 1;
* down_read(&mm->mmap_sem);
* do_something()
* get_user_pages_locked(tsk, mm, ..., pages, &locked);
* if (locked)
* up_read(&mm->mmap_sem);

So I understood it as a way to reduce mmap_sem hold time by doing a try
first. Did I understand that comment wrong?

Honza

> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Jan Kara <jack@suse.cz>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andrea Arcangeli <aarcange@redhat.com>
> ---
>
> b/mm/frame_vector.c | 9 +++------
> 1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff -puN mm/frame_vector.c~remove-get_user_pages_locked-user mm/frame_vector.c
> --- a/mm/frame_vector.c~remove-get_user_pages_locked-user 2015-11-16 12:35:35.282169293 -0800
> +++ b/mm/frame_vector.c 2015-11-16 12:35:35.285169429 -0800
> @@ -40,7 +40,6 @@ int get_vaddr_frames(unsigned long start
> struct vm_area_struct *vma;
> int ret = 0;
> int err;
> - int locked;
>
> if (nr_frames == 0)
> return 0;
> @@ -49,7 +48,6 @@ int get_vaddr_frames(unsigned long start
> nr_frames = vec->nr_allocated;
>
> down_read(&mm->mmap_sem);
> - locked = 1;
> vma = find_vma_intersection(mm, start, start + 1);
> if (!vma) {
> ret = -EFAULT;
> @@ -58,8 +56,8 @@ int get_vaddr_frames(unsigned long start
> if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
> vec->got_ref = true;
> vec->is_pfns = false;
> - ret = get_user_pages_locked(current, mm, start, nr_frames,
> - write, force, (struct page **)(vec->ptrs), &locked);
> + ret = get_user_pages(current, mm, start, nr_frames,
> + write, force, (struct page **)(vec->ptrs), NULL);
> goto out;
> }
>
> @@ -87,8 +85,7 @@ int get_vaddr_frames(unsigned long start
> vma = find_vma_intersection(mm, start, start + 1);
> } while (vma && vma->vm_flags & (VM_IO | VM_PFNMAP));
> out:
> - if (locked)
> - up_read(&mm->mmap_sem);
> + up_read(&mm->mmap_sem);
> if (!ret)
> ret = -EFAULT;
> if (ret > 0)
> _
--
Jan Kara <jack@suse.com>
SUSE Labs, CR


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