Re: [PATCH 18/41] mm/khugepaged: write-lock VMA while collapsing a huge page

[Jann Horn <jannh@google.com>]

On Wed, Jan 18, 2023 at 10:40 AM Michal Hocko <mhocko@suse.com> wrote:
> On Tue 17-01-23 21:28:06, Jann Horn wrote:
> > On Tue, Jan 17, 2023 at 4:25 PM Michal Hocko <mhocko@suse.com> wrote:
> > > On Mon 09-01-23 12:53:13, Suren Baghdasaryan wrote:
> > > > Protect VMA from concurrent page fault handler while collapsing a huge
> > > > page. Page fault handler needs a stable PMD to use PTL and relies on
> > > > per-VMA lock to prevent concurrent PMD changes. pmdp_collapse_flush(),
> > > > set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will
> > > > not be detected by a page fault handler without proper locking.
> > >
> > > I am struggling with this changelog. Maybe because my recollection of
> > > the THP collapsing subtleties is weak. But aren't you just trying to say
> > > that the current #PF handling and THP collapsing need to be mutually
> > > exclusive currently so in order to keep that assumption you have mark
> > > the vma write locked?
> > >
> > > Also it is not really clear to me how that handles other vmas which can
> > > share the same thp?
> >
> > It's not about the hugepage itself, it's about how the THP collapse
> > operation frees page tables.
> >
> > Before this series, page tables can be walked under any one of the
> > mmap lock, the mapping lock, and the anon_vma lock; so when khugepaged
> > unlinks and frees page tables, it must ensure that all of those either
> > are locked or don't exist. This series adds a fourth lock under which
> > page tables can be traversed, and so khugepaged must also lock out that one.
> >
> > There is a codepath in khugepaged that iterates through all mappings
> > of a file to zap page tables (retract_page_tables()), which locks each
> > visited mm with mmap_write_trylock() and now also does
> > vma_write_lock().
>
> OK, I see. This would be a great addendum to the changelog.
>
> > I think one aspect of this patch that might cause trouble later on, if
> > support for non-anonymous VMAs is added, is that retract_page_tables()
> > now does vma_write_lock() while holding the mapping lock; the page
> > fault handling path would probably take the locks the other way
> > around, leading to a deadlock? So the vma_write_lock() in
> > retract_page_tables() might have to become a trylock later on.
>
> This, right?
> #PF                     retract_page_tables
> vma_read_lock
>                         i_mmap_lock_write
> i_mmap_lock_read
>                         vma_write_lock
>
>
> I might be missing something but I have only found huge_pmd_share to be
> called from the #PF path. That one should be safe as it cannot be a
> target for THP. Not that it would matter much because such a dependency
> chain would be really subtle.

Oops, yeah. Now that I'm looking closer I also don't see a path from
the #PF path to i_mmap_lock_read. Sorry for sending you on a wild
goose chase.