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SubjectRe: [PATCH 02/37] mm, frame_vector: do not use get_user_pages_locked()
On Wed, Nov 18, 2015 at 01:29:38PM +0100, Jan Kara wrote:
> 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?

That is correct. get_user_pages_locked should not be downgraded to
get_user_pages or it can actually break userfaultfd, as userfaultfd
needs to be allowed to drop the mmap_sem within handle_mm_fault, to
function correctly. Furthermore get_user_pages_locked allows for
higher SMP scalability as it can take advantage of the optimized
pagecache code that drops the mmap_sem before waiting for I/O (and
then it retries the page fault after the I/O is complete).

The only case where get_user_pages without FOLL_FAULT_ALLOW_RETRY
makes any sense is the get_dump_page case which is actually not using
get_user_pages in the first place, so ideally get_user_pages should be
obsoleted and dropped.

Thanks,
Andrea


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