Re: [PATCH 0/5] mm improvements

[Nikita Danilov <Nikita@xxxxxxxxxxx>]

Hugh Dickins writes:
 > 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.

Ah, I see.

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

May be one can just call try_to_unmap() from shmem_writepage()?

 > 
 > Hugh
 > 

Nikita.
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