Re: [PATCH 1/2] mm: Take all anon_vma locks in anon_vma_lock

[Andrea Arcangeli]

> > Btw, Mel's patch doesn't really match the description of 2/2. 2/2 says
> > that all pages must always be findable in rmap. Mel's patch seems to
> > explicitly say "we want to ignore that thing that is busy for execve". Are
> > we just avoiding a BUG_ON()? Is perhaps the BUG_ON() buggy?
> 
> I have no good answer to this question.
> 
> Mel?  Andrea?

If try_to_unmap is allowed to establish the migration pte, then such
pte has to remain reachable through rmap_walk at all times after that,
or migration_entry_wait will crash because it notices the page has
been migrated already (PG_lock not set) but there is still a migration
pte established. (remove_migration_pte like split_huge_page isn't
allowed to fail finding all migration ptes mapping a page during the
rmap walk)

It's not false positive BUG_ON if that's what you mean, removing the
BUG_ON would still lead to infinite hang waiting on a migration pte
that shouldn't be there anymore.
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