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Subject: Re: [PATCH 00/11] V4: rwsem changes + down_read_critical() proposal
From: Peter Zijlstra <peterz@infradead.org>
To: Michel Lespinasse <walken@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
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Date: Tue, 25 May 2010 11:27:55 +0200
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On Tue, 2010-05-25 at 02:12 -0700, Michel Lespinasse wrote:
> On Tue, May 25, 2010 at 1:47 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > So what happened to those patches that dropped mmap_sem during I/O?
> 
> Yes, we do have patches trying to release the mmap_sem when a page
> fault for a file backed VMA blocks on accessing the corresponding
> file. We have not given up on these, and we intend to try submitting
> them again. However, these patches do *not* address the case of a page
> fault blocking while trying to get a free page (i.e. when you get
> under high memory pressure).

But I guess they could, right? Simply make the allocation under mmap_sem
be __GFP_HARDWALL|__GFP_HIGHMEM|__GFP_MOVABLE__GFP_NOWARN or
(GFP_HUGHUSER_MOVABLE & ~(__GFP_WAIT|__GFP_IO|__GFP_FS))|__GFP_NOWARN

and drop the mmap_sem when that fails.

> > I really don't like people tinkering with the lock implementations like
> > this. Nor do I like the naming, stats are in no way _critical_.
> 
> Critical here refers to the fact that you're not allowed to block
> while holding the unfairly acquired rwsem.

We usually call that atomic, your 0/n patch didn't explain any of that.

Also, do you really think doing something like:

        /*
         * Check the vma index is within the range and do
         * sequential scan until m_index.
         */
        vma = NULL;
        if ((unsigned long)l < mm->map_count) {
                vma = mm->mmap;
                while (l-- && vma)
                        vma = vma->vm_next;
                goto out;
        }

with preemption disabled is a _good_ thing?

People were talking about raising our vma limit of 64k...