Re: mm: VM_BUG_ON_PAGE(PageTail(page)) in mbind

[Kirill A. Shutemov]

On Tue, Jan 26, 2016 at 12:48:23PM -0800, Andrew Morton wrote:
> On Tue, 26 Jan 2016 22:28:29 +0200 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote:
> 
> > The patch below fixes the issue for me, but this bug makes me wounder how
> > many bugs like this we have in kernel... :-/
> > 
> > Looks like we are too permissive about which VMA is migratable:
> > vma_migratable() filters out VMA by VM_IO and VM_PFNMAP.
> > I think VM_DONTEXPAND also correlate with VMA which cannot be migrated.
> > 
> > $ git grep VM_DONTEXPAND drivers | grep -v '\(VM_IO\|VM_PFNMAN\)' | wc -l 
> > 33
> > 
> > Hm.. :-|
> > 
> > It worth looking on them closely... And I wouldn't be surprised if some
> > VMAs without all of these flags are not migratable too.
> > 
> > Sigh.. Any thoughts?
> 
> Sigh indeed.  I think that both VM_DONTEXPAND and VM_DONTDUMP are
> pretty good signs that mbind() should not be mucking with this vma.  If
> such a policy sometimes results in mbind failing to set a policy then
> that's not a huge loss - something runs a bit slower maybe.
> 
> I mean, we only really expect mbind() to operate against regular old
> anon/pagecache memory, yes?

Well, it can work fine too if driver itself uses page tables to find out
which pages it should to operate on. I don't think it's a common case.

-- 
 Kirill A. Shutemov