Re: hard question re: swap cache

[Hugh Dickins <hugh@veritas.com>]

On Tue, 27 May 2003, Carl Spalletta wrote:
> Assume a shared, anonymous page is referenced by a set of
> processes a,b,c,d,e and the page is marked present in the
> page tables of each process.  Assume then that the page is
> marked for swapout in the pagetables of 'a'. A swap slot is
> filled with a copy of the page, but it is still present in
> memory.
> ....
> Then say b,c,d and e in that order have the page swapped out.
> Either the page is copied to the page slot for each swapout
> or it _must_ be copied on the last swap (when the page usage
> counter goes to zero) else the modifications made by b,c,d,e
> will be lost.

I'm not certain I understand your question (in particular, I don't
understand the page being copied to a page slot), but I might have
your answer.

Observe that mm/mmap.c:do_mmap_pgoff uses mm/shmem.c:shmem_zero_setup
for a shared anonymous mapping.  That creates a tmpfs object to back
the mapping, so its pages are not _directly_ backed by swap.

Under memory pressure, shmem_writepage gets called, which translates
(well, akpm's superb technical term for this is "swizzles") the page
to swap, and then later shmem_getpage may bring it back in.  Note
the BUG_ON(page_mapped(page)) in shmem_writepage, which gives the
assurance I think you're looking for.

Hugh

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