Re: Dirty/Access bits vs. page content

[Linus Torvalds <torvalds@linux-foundation.org>]

On Fri, Apr 25, 2014 at 5:01 AM, Hugh Dickins <hughd@google.com> wrote:
>
> Two, Ben said earlier that he's more worried about users of
> unmap_mapping_range() than concurrent munmap(); and you said
> earlier that you would almost prefer to have some special lock
> to serialize with page_mkclean().
>
> Er, i_mmap_mutex.
>
> That's what unmap_mapping_range(), and page_mkclean()'s rmap_walk,
> take to iterate over the file vmas.  So perhaps there's no race at all
> in the unmap_mapping_range() case.  And easy (I imagine) to fix the
> race in Dave's racewrite.c use of MADV_DONTNEED: untested patch below.

Hmm. unmap_mapping_range() is just abotu the only thing that _does_
take i_mmap_mutex. unmap_single_vma() does it for
is_vm_hugetlb_page(), which is a bit confusing. And normally we only
take it for the actual final vma link/unlink, not for the actual
traversal. So we'd have to change that all quite radically (or we'd
have to drop and re-take it).

So I'm not quite convinced. Your simple patch looks simple and should
certainly fix DaveH's test-case, but then leaves munmap/exit as a
separate thing to fix. And I don't see how to do that cleanly (it
really looks like "we'll just have to take that semaphore again
separately).

i_mmap_mutex is likely not contended, but we *do* take it for private
mappings too (and for read-only ones), so this lock is actually much
more common than the dirty shared mapping.

So I think I prefer my patch, even if that may be partly due to just
it being mine ;)

          Linus

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