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Date: Tue, 08 Apr 2008 18:59:19 +0200
From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
To: Nick Piggin <nickpiggin@...oo.com.au>
Cc: Eric Dumazet <dada1@...mosbay.com>, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH 1/2] futex: rely on get_user_pages() for shared
futexes
On Tue, 2008-04-08 at 21:40 +1000, Nick Piggin wrote:
> On Saturday 05 April 2008 06:33, Peter Zijlstra wrote:
> > On the way of getting rid of the mmap_sem requirement for shared futexes,
> > start by relying on get_user_pages().
> >
> > This requires we get the page associated with the key, and put the page
> > when we're done with it.
>
> Hi Peter,
>
> Cool.
>
> I'm all for removing mmap_sem requirement from shared futexes...
> Are there many apps which make a non-trivial use of them I wonder?
No idea. I've heard some stories, but I've never seen any code..
> I guess it will help legacy (pre-FUTEX_PRIVATE) usespaces in
> performance too, though.
Yeah, that was one of the motivations for this patch.
> What I'm worried about with this is invalidate or truncate races.
> With direct IO, it obviously doesn't matter because the only
> requirement is that the page existed at the address at some point
> during the syscall...
>
> So I'd really like you to not carry the page around in the key, but
> just continue using the same key we have now. Also, lock the page
> and ensure it hasn't been truncated before taking the inode from the
> key and incrementing its count (page lock's extra atomics should be
> more or less cancelled out by fewer mmap_sem atomic ops).
>
> get_futex_key should look something like this I would have thought:?
Does look nicer, will have to ponder it a bit though.
I must admit to not fully understanding why we take inode/mm references
in the key anyway as neither will stop unmapping the futex.
Will make this into a nice patch.. thanks!
> BTW. I like that it removes a lot of fshared crap from around
> the place. And also this is a really good user of fast_gup
> because I guess it should usually be faulted in. The problem is
> that this could be a little more expensive for architectures that
> don't implement fast_gup. Though most should be able to.
Yeah, if that really becomes a problem (but I doubt it will) we could
possibly make the old and new scheme work based on ARCH_HAVE_PTE_SPECIAL
or something ugly like that.
> @@ -191,7 +191,6 @@ static int get_futex_key(u32 __user *uad
> {
> unsigned long address = (unsigned long)uaddr;
> struct mm_struct *mm = current->mm;
> - struct vm_area_struct *vma;
> struct page *page;
> int err;
>
> @@ -210,27 +209,26 @@ static int get_futex_key(u32 __user *uad
> * Note : We do have to check 'uaddr' is a valid user address,
> * but access_ok() should be faster than find_vma()
> */
> - if (!fshared) {
> + if (likely(!fshared)) {
> if (unlikely(!access_ok(VERIFY_WRITE, uaddr, sizeof(u32))))
> return -EFAULT;
> key->private.mm = mm;
> key->private.address = address;
> return 0;
> }
> - /*
> - * The futex is hashed differently depending on whether
> - * it's in a shared or private mapping. So check vma first.
> - */
> - vma = find_extend_vma(mm, address);
> - if (unlikely(!vma))
> - return -EFAULT;
> -
> - /*
> - * Permissions.
> - */
> - if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
> - return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;
>
> +again:
> + err = fast_gup(address, 1, 0, &page);
> + if (err < 0)
> + return err;
> +
> + lock_page(page);
> + if (!page->mapping) { /* PageAnon pages shouldn't get caught here */
> + unlock_page(page);
> + put_page(page);
> + goto again;
> + }
> +
> /*
> * Private mappings are handled in a simple way.
> *
> @@ -240,38 +238,19 @@ static int get_futex_key(u32 __user *uad
> * VM_MAYSHARE here, not VM_SHARED which is restricted to shared
> * mappings of _writable_ handles.
> */
> - if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
> - key->both.offset |= FUT_OFF_MMSHARED; /* reference taken on mm
> *
> /
> + if (PageAnon(page)) {
> + key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
> key->private.mm = mm;
> key->private.address = address;
> - return 0;
> - }
> -
> - /*
> - * Linear file mappings are also simple.
> - */
> - key->shared.inode = vma->vm_file->f_path.dentry->d_inode;
> - key->both.offset |= FUT_OFF_INODE; /* inode-based key. */
> - if (likely(!(vma->vm_flags & VM_NONLINEAR))) {
> - key->shared.pgoff = (((address - vma->vm_start) >> PAGE_SHIFT)
> - + vma->vm_pgoff);
> - return 0;
> - }
> -
> - /*
> - * We could walk the page table to read the non-linear
> - * pte, and get the page index without fetching the page
> - * from swap. But that's a lot of code to duplicate here
> - * for a rare case, so we simply fetch the page.
> - */
> - err = get_user_pages(current, mm, address, 1, 0, 0, &page, NULL);
> - if (err >= 0) {
> - key->shared.pgoff =
> - page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
> - put_page(page);
> - return 0;
> + } else {
> + key->both.offset |= FUT_OFF_INODE; /* inode-based key. */
> + key->shared.inode = page->mapping->inode;
> + key->shared.pgoff = page->index;
> }
> - return err;
> +out:
> + unlock_page(page);
> + put_page(page);
> + return 0;
> }
>
> /*
>
--
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