Re: [PATCH] uprobes: __replace_page() avoid BUG in munlock_vma_page()

[Song Liu]

> On Aug 17, 2020, at 12:17 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> On Mon, 17 Aug 2020, Song Liu wrote:
>>> On Aug 16, 2020, at 1:44 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>>> 
>>> syzbot crashed on the VM_BUG_ON_PAGE(PageTail) in munlock_vma_page(),
>>> when called from uprobes __replace_page().  Which of many ways to fix it?
>>> Settled on not calling when PageCompound (since Head and Tail are equals
>>> in this context, PageCompound the usual check in uprobes.c, and the prior
>>> use of FOLL_SPLIT_PMD will have cleared PageMlocked already).
>>> 
>>> Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
>>> Fixes: 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
>>> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
>>> Cc: stable@xxxxxxxxxxxxxxx # v5.4+
>>> ---
>>> This one is not a 5.9-rc regression, but still good to fix.
>>> 
>>> kernel/events/uprobes.c |    2 +-
>>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>> 
>>> --- v5.9-rc/kernel/events/uprobes.c	2020-08-12 19:46:50.851196584 -0700
>>> +++ linux/kernel/events/uprobes.c	2020-08-16 13:18:35.292821674 -0700
>>> @@ -205,7 +205,7 @@ static int __replace_page(struct vm_area
>>> 		try_to_free_swap(old_page);
>>> 	page_vma_mapped_walk_done(&pvmw);
>>> 
>>> -	if (vma->vm_flags & VM_LOCKED)
>>> +	if ((vma->vm_flags & VM_LOCKED) && !PageCompound(old_page))
>> 
>> Do we need munlock_vma_page() for THP page head? 
> 
> No: as the commit message says "the prior use of FOLL_SPLIT_PMD
> will have cleared PageMlocked already" - our THP implementation
> has difficulty supporting Mlocked consistently when the huge page is
> somewhere mapped by ptes, so one of the things that __split_huge_pmd()
> does is clear_page_mlock(), then PageDoubleMap will prevent Mlocked
> being set again once GUP has brought the old_page pte back in.
> 
> But if you'd prefer us to munlock_vma_page(compound_head(old_page))
> instead, I can certainly change the patch: it's one of the options
> I considered, but couldn't quite bring myself to do it that way,
> knowing that actually it would never find PageMlocked set.  (If
> PageMlocked were allowed on tail pages, I'd have used a PageMlocked
> test instead of the PageCompound one: I spent nearly an hour
> bikeshedding the alternatives here!)
> 
> (One day I must remind myself of when munlock_vma_page() should be
> used, versus when clear_page_mlock() should be used: I think it comes
> down to a choice of which stats get incremented, but I may also be
> forgetting something more important: anyway, no obvious reason to
> depart from the munlock_vma_page() that's always been used here.)

Thanks a lot for the explanation!

Acked-by: Song Liu <songliubraving@xxxxxx>