Re: kernel 3.0: BUG: soft lockup: find_get_pages+0x51/0x110

[Nai Xia]

Hi Andrea,

On Mon, Oct 17, 2011 at 7:54 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> On Thu, Oct 13, 2011 at 04:30:09PM -0700, Hugh Dickins wrote:
>> mremap's down_write of mmap_sem, together with i_mmap_mutex/lock,
>> and pagetable locks, were good enough before page migration (with its
>> requirement that every migration entry be found) came in; and enough
>> while migration always held mmap_sem.  But not enough nowadays, when
>> there's memory hotremove and compaction: anon_vma lock is also needed,
>> to make sure a migration entry is not dodging around behind our back.
>
> For things like migrate and split_huge_page, the anon_vma layer must
> guarantee the page is reachable by rmap walk at all times regardless
> if it's at the old or new address.
>
> This shall be guaranteed by the copy_vma called by move_vma well
> before move_page_tables/move_ptes can run.
>
> copy_vma obviously takes the anon_vma lock to insert the new "dst" vma
> into the anon_vma chains structures (vma_link does that). That before
> any pte can be moved.
>
> Because we keep two vmas mapped on both src and dst range, with
> different vma->vm_pgoff that is valid for the page (the page doesn't
> change its page->index) the page should always find _all_ its pte at
> any given time.
>
> There may be other variables at play like the order of insertion in
> the anon_vma chain matches our direction of copy and removal of the
> old pte. But I think the double locking of the PT lock should make the
> order in the anon_vma chain absolutely irrelevant (the rmap_walk
> obviously takes the PT lock too), and furthermore likely the
> anon_vma_chain insertion is favorable (the dst vma is inserted last
> and checked last). But it shouldn't matter.

I happened to be reading these code last week.

And I do think this order matters, the reason is just quite similar why we
need i_mmap_lock in move_ptes():
If rmap_walk goes dst--->src, then when it first look into dst, ok, the
pte is not there, and it happily skip it and release the PTL.
Then just before it look into src, move_ptes() comes in, takes the locks
and moves the pte from src to dst. And then when rmap_walk() look
into src,  it will find an empty pte again. The pte is still there,
but rmap_walk() missed it !

IMO, this can really happen in case of vma_merge() succeeding.
Imagine that src vma is lately faulted and in anon_vma_prepare()
it got a same anon_vma with an existing vma ( named evil_vma )through
find_mergeable_anon_vma().  This can potentially make the vma_merge() in
copy_vma() return with evil_vma on some new relocation request. But src_vma
is really linked _after_  evil_vma/new_vma/dst_vma.
In this way, the ordering protocol  of anon_vma chain is broken.
This should be a rare case because I think in most cases
if two VMAs can reusable_anon_vma() they were already merged.

How do you think  ?

And If my reasoning is sound and this bug is really triggered by it
Hugh's first patch should be the right fix :)


Regards,

Nai Xia

>
> Another thing could be the copy_vma vma_merge branch succeeding
> (returning not NULL) but I doubt we risk to fall into that one. For
> the rmap_walk to be always working on both the src and dst
> vma->vma_pgoff the pgoff must be different so we can't possibly be ok
> if there's just 1 vma covering the whole range. I exclude this could
> be the case because the pgoff passed to copy_vma is different than the
> vma->vm_pgoff given to copy_vma, so vma_merge can't possibly succeed.
>
> Yet another point to investigate is the point where we teardown the
> old vma and we leave the new vma generated by copy_vma
> established. That's apparently taken care of by do_munmap in move_vma
> so that shall be safe too as munmap is safe in the first place.
>
> Overall I don't think this patch is needed and it seems a noop.
>
>> It appears that Mel's a8bef8ff6ea1 "mm: migration: avoid race between
>> shift_arg_pages() and rmap_walk() during migration by not migrating
>> temporary stacks" was actually a workaround for this in the special
>> common case of exec's use of move_pagetables(); and we should probably
>> now remove that VM_STACK_INCOMPLETE_SETUP stuff as a separate cleanup.
>
> I don't think this patch can help with that, the problem of execve vs
> rmap_walk is that there's 1 single vma existing for src and dst
> virtual ranges while execve runs move_page_tables. So there is no
> possible way that rmap_walk will be guaranteed to find _all_ ptes
> mapping a page if there's just one vma mapping either the src or dst
> range while move_page_table runs. No addition of locking whatsoever
> can fix that bug because we miss a vma (well modulo locking that
> prevents rmap_walk to run at all, until we're finished with execve,
> which is more or less what VM_STACK_INCOMPLETE_SETUP does...).
>
> The only way is to fix this is prevent migrate (or any other rmap_walk
> user that requires 100% reliability from the rmap layer, for example
> swap doesn't require 100% reliability and can still run and gracefully
> fail at finding the pte) while we're moving pagetables in execve. And
> that's what Mel's above mentioned patch does.
>
> The other way to fix that bug that I implemented was to do copy_vma in
> execve, so that we still have both src and dst ranges of
> move_page_tables covered by 2 (not 1) vma, each with the proper
> vma->vm_pgoff, so my approach fixed that bug as well (but requires a
> vma allocation in execve so it was dropped in favor of Mel's patch
> which is totally fine with as both approaches fixes the bug equally
> well, even if now we've to deal with this special case of sometime
> rmap_walk having false negatives if the vma_flags is set, and the
> important thing is that after VM_STACK_INCOMPLETE_SETUP has been
> cleared it won't ever be set again for the whole lifetime of the vma).
>
> I may be missing something, I did a short review so far, just so the
> patch doesn't get merged if not needed. I mean I think it needs a bit
> more looks on it... The fact the i_mmap_mutex was taken but the
> anon_vma lock was not taken (while in every other place they both are
> needed) certainly makes the patch look correct, but that's just a
> misleading coincidence I think.
>
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