linux-kernel - Re: [PATCH 1/3] vrange: Add vrange syscall and handle splitting/merging and marking vmas

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Date:	Mon, 17 Mar 2014 10:43:39 +0100
From:	Jan Kara <jack@...e.cz>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>, Dave Hansen <dave@...1.net>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	Dhaval Giani <dgiani@...illa.com>, Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 1/3] vrange: Add vrange syscall and handle
 splitting/merging and marking vmas

On Mon 17-03-14 10:21:18, Jan Kara wrote:
> On Fri 14-03-14 11:33:31, John Stultz wrote:
> > +	for (;;) {
> > +		unsigned long new_flags;
> > +		pgoff_t pgoff;
> > +		unsigned long tmp;
> > +
> > +		if (!vma)
> > +			goto out;
> > +
> > +		if (vma->vm_flags & (VM_SPECIAL|VM_LOCKED|VM_MIXEDMAP|
> > +					VM_HUGETLB))
> > +			goto out;
> > +
> > +		/* We don't support volatility on files for now */
> > +		if (vma->vm_file) {
> > +			ret = -EINVAL;
> > +			goto out;
> > +		}
> > +
> > +		new_flags = vma->vm_flags;
> > +
> > +		if (start < vma->vm_start) {
> > +			start = vma->vm_start;
> > +			if (start >= end)
> > +				goto out;
> > +		}
  One more question: This seems to silently skip any holes between VMAs. Is
that really intended? I'd expect that marking unmapped range as volatile /
non-volatile should return error... In any case what happens should be
defined in the description.

								Honza

> > +		tmp = vma->vm_end;
> > +		if (end < tmp)
> > +			tmp = end;
> > +
> > +		switch (mode) {
> > +		case VRANGE_VOLATILE:
> > +			new_flags |= VM_VOLATILE;
> > +			break;
> > +		case VRANGE_NONVOLATILE:
> > +			new_flags &= ~VM_VOLATILE;
> > +		}
> > +
> > +		pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
> > +		prev = vma_merge(mm, prev, start, tmp, new_flags,
> > +					vma->anon_vma, vma->vm_file, pgoff,
> > +					vma_policy(vma));
> > +		if (prev)
> > +			goto success;
> > +
> > +		if (start != vma->vm_start) {
> > +			ret = split_vma(mm, vma, start, 1);
> > +			if (ret)
> > +				goto out;
> > +		}
> > +
> > +		if (tmp != vma->vm_end) {
> > +			ret = split_vma(mm, vma, tmp, 0);
> > +			if (ret)
> > +				goto out;
> > +		}
> > +
> > +		prev = vma;
> > +success:
> > +		vma->vm_flags = new_flags;
> > +		*purged = lpurged;
> > +
> > +		/* update count to distance covered so far*/
> > +		count = tmp - orig_start;
> > +
> > +		if (prev && start < prev->vm_end)
>   In which case 'prev' can be NULL? And when start >= prev->vm_end? In all
> the cases I can come up with this condition seems to be true...
> 
> > +			start = prev->vm_end;
> > +		if (start >= end)
> > +			goto out;
> > +		if (prev)
>   Ditto regarding 'prev'...
> 
> > +			vma = prev->vm_next;
> > +		else	/* madvise_remove dropped mmap_sem */
> > +			vma = find_vma(mm, start);
>   The comment regarding madvise_remove() looks bogus...
> 
> > +	}
> > +out:
> > +	up_read(&mm->mmap_sem);
> > +
> > +	/* report bytes successfully marked, even if we're exiting on error */
> > +	if (count)
> > +		return count;
> > +
> > +	return ret;
> > +}
> > +
> > +SYSCALL_DEFINE4(vrange, unsigned long, start,
> > +		size_t, len, int, mode, int __user *, purged)
> > +{
> > +	unsigned long end;
> > +	struct mm_struct *mm = current->mm;
> > +	ssize_t ret = -EINVAL;
> > +	int p = 0;
> > +
> > +	if (start & ~PAGE_MASK)
> > +		goto out;
> > +
> > +	len &= PAGE_MASK;
> > +	if (!len)
> > +		goto out;
> > +
> > +	end = start + len;
> > +	if (end < start)
> > +		goto out;
> > +
> > +	if (start >= TASK_SIZE)
> > +		goto out;
> > +
> > +	if (purged) {
> > +		/* Test pointer is valid before making any changes */
> > +		if (put_user(p, purged))
> > +			return -EFAULT;
> > +	}
> > +
> > +	ret = do_vrange(mm, start, end, mode, &p);
> > +
> > +	if (purged) {
> > +		if (put_user(p, purged)) {
> > +			/*
> > +			 * This would be bad, since we've modified volatilty
> > +			 * and the change in purged state would be lost.
> > +			 */
> > +			WARN_ONCE(1, "vrange: purge state possibly lost\n");
> > +		}
> > +	}
> > +
> > +out:
> > +	return ret;
> > +}
> 								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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