Re: [PATCH 0/5] mm improvements

[Hugh Dickins <hugh@xxxxxxxxxxx>]

On Wed, 4 Feb 2004, Nikita Danilov wrote:
> 
> 4. I found that shmem_writepage() has BUG_ON(page_mapped(page))
> check. Its removal had no effect, and I am not sure why the check was
> there at all.

Sorry, that BUG_ON is there for very good reason.  It's no disgrace
that your testing didn't notice the effect of passing a mapped page
down to shmem_writepage, but it is a serious breakage of tmpfs.

I'd have to sit here thinking awhile to remember if there are further
reasons why it's a no-no.  But the reason that springs to mind is it
breaks the semantics of a tmpfs file mapped shared into different mms.
shmem_writepage changes the tmpfs-file identity of the page to swap
identity: so if it's unmapped later, the instances would then become
private (to be COWed) instead of shared.

If you go the writepage-while-mapped route (more general gotchas?
I forget), you'll have to make an exception for shmem_writepage.

Hugh

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