Re: [PATCH v3 7/8] mm/mempolicy: use a standard migration target allocation callback

[Hugh Dickins <hughd@google.com>]

On Thu, 8 Oct 2020, Mike Kravetz wrote:
> On 10/7/20 8:21 PM, Hugh Dickins wrote:
> > 
> > Mike, j'accuse... your 5.7 commit c0d0381ade79 ("hugetlbfs:
> > use i_mmap_rwsem for more pmd sharing synchronization"), in which
> > unmap_and_move_huge_page() now passes the TTU_RMAP_LOCKED flag to
> > try_to_unmap(), because it's already holding mapping->i_mmap_rwsem:
> > but that is not the right lock to secure an anon_vma lookup.
> 
> Thanks Hugh!  Your analysis is correct and the code in that commit is
> not correct.  I was so focused on the file mapping case, I overlooked
> (actually introduced) this issue for anon mappings.
> 
> Let me verify that this indeed is the root cause.  However, since
> move_pages12 migrated anon hugetlb pages it certainly does look to be
> the case.
> 
> > I intended to send a patch, passing TTU_RMAP_LOCKED only in the
> > !PageAnon case (and, see vma_adjust(), anon_vma lock conveniently
> > nests inside i_mmap_rwsem); but then wondered if i_mmap_rwsem was
> > needed in that case or not, so looked deeper into c0d0381ade79.
> > 
> > Hmm, not even you liked it!  But the worst of it looks simply
> > unnecessary to me, and I hope can be deleted - but better by you
> > than by me (in particular, you were trying to kill 1) and 2) birds
> > with one stone, and I've always given up on understanding hugetlb's
> > reservations: I suspect that side of it is irrelevant here,
> > but I wouldn't pretend to be sure).
> > 
> > How could you ever find a PageAnon page in a vma_shareable() area?
> > 
> > It is all rather confusing (vma_shareable() depending on VM_MAYSHARE,
> > whereas is_cow_mapping() on VM_SHARED and VM_MAYWRITE: they have to
> > be studied together with do_mmap()'s 
> > 			vm_flags |= VM_SHARED | VM_MAYSHARE;
> > 			if (!(file->f_mode & FMODE_WRITE))
> > 				vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
> > 
> > (And let me add to the confusion by admitting that, prior to 3.15's
> > cda540ace6a1 "mm: get_user_pages(write,force) refuse to COW in
> > shared areas", maybe it was possible to find a PageAnon there.)
> > 
> > But my belief (best confirmed by you running your tests with a
> > suitably placed BUG_ON or WARN_ON) is that you'll never find a
> > PageAnon in a vma_shareable() area, so will never need try_to_unmap()
> > to unshare a pagetable in the PageAnon case, so won't need i_mmap_rwsem
> > for PageAnon there, and _get_hugetlb_page_mapping() (your function that
> > deduces an address_space from an anon_vma) can just be deleted.
> 
> Yes, it is confusing.  Let me look into this.  I would be really happy
> to delete that ugly function.
> 
> > (And in passing, may I ask what hugetlb_page_mapping_lock_write()'s
> > hpage->_mapcount inc and dec are for?  You comment it as a hack,
> > but don't explain what needs that hack, and I don't see it.)
> 
> We are trying to lock the mapping (mapping->i_mmap_rwsem).  We know
> mapping is valid, because we obtained it from page_mapping() and it
> will remain valid because we have the page locked.  Page needs to be
> unlocked to unmap.  However, we have to drop page lock in order to
> acquire i_mmap_rwsem.  Once we drop page lock, mapping could become
> invalid.  So, the code code artifically incs mapcount so that mapping
> will remain valid when upmapping page.

No, unless you can point me to some other hugetlbfs-does-it-differently
(and I didn't see it there in that commit), raising _mapcount does not
provide any such protection; but does add the possiblility of a
"BUG: Bad page cache" and leak from unaccount_page_cache_page().

Earlier in the day I was trying to work out what to recommend instead,
but had to turn aside to something else: I'll try again tomorrow.

It's a problem I've faced before in tmpfs, keeping a hold on the
mapping while page lock is dropped.  Quite awkward: igrab() looks as
if it's the right thing to use, but turns out to give no protection
against umount.  Last time around, I ended up with a stop_eviction
count in the shmem inode, which shmem_evict_inode() waits on if
necessary.  Something like that could be done for hugetlbfs too,
but I'd prefer to do it without adding extra, if there is a way.

> 
> As mentioned above, I hope all this can be removed.

If you continue to nest page lock inside i_mmap_rwsem for hugetlbfs,
then I think that part of hugetlb_page_mapping_lock_write() has to
remain.  I'd much prefer that hugetlbfs did not reverse the usual
nesting, but accept that you had reasons for doing it that way.

Hugh