linux-kernel - Re: [PATCH] mm: Ignore VM_SOFTDIRTY on VMA merging, v2

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Date:	Fri, 24 Jan 2014 01:45:05 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Pavel Emelyanov <xemul@...allels.com>,
	Mel Gorman <mgorman@...e.de>, gnome@...t.net,
	grawoc@...krefraction.com, alan@...rguk.ukuu.org.uk,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	bugzilla-daemon@...zilla.kernel.org
Subject: Re: [PATCH] mm: Ignore VM_SOFTDIRTY on VMA merging, v2

On Thu, Jan 23, 2014 at 01:02:35PM -0800, Andrew Morton wrote:
> On Thu, 23 Jan 2014 19:14:45 +0400 Cyrill Gorcunov <gorcunov@...il.com> wrote:
> 
> > VM_SOFTDIRTY bit affects vma merge routine: if two VMAs has all
> > bits in vm_flags matched except dirty bit the kernel can't longer
> > merge them and this forces the kernel to generate new VMAs instead.
> 
> Do you intend to alter the brk() and binprm code to set VM_SOFTDIRTY?

brk() will be "dirtified" now with this merge fix.
brk
  do_brk
    out:
	...
	vma->vm_flags |= VM_SOFTDIRTY;

this will work even if vma get merged, the problem was that earlier
we tried to merge without VM_SOFTDIRTY flag. And matcher failed.

do_brk
  flags = VM_DATA_DEFAULT_FLAGS | VM_ACCOUNT | mm->def_flags;
	vma = vma_merge(mm, prev, addr, addr + len, flags,
					NULL, NULL, pgoff, NULL);
	if (vma)
		goto out;
...
out:
	...
	vma->vm_flags |= VM_SOFTDIRTY;

That said I'm not really sure now if I should alert @flags in code above.
Should I add VM_SOFTDIRTY into @flags for clarity?

Same for binprm -- the vma allocated for bprm->vma is dirtified
__bprm_mm_init
  vma->vm_flags = VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;

then setup_arg_pages calls mprotect_fixup with @vm_flags having dirty bit
set thus it'll be propagated to vma

mprotect_fixup
  ...
  vma->vm_flags = newflags;

the @newflags will have dirty bit set from caller code.

Or you mean something else which I'm missing?
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