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SubjectRe: [PATCH 2.6.21] cramfs: add cramfs Linear XIP
Jörn Engel wrote:
> Nitbit: Sooner or later the point/unpoint should get replaced by
> something with page granularity. Something also needs to keep lists of
> mapped pages and invalidate them whenever the device is written to.
> That could be done in the filesystem or device driver. I believe the
> device driver would be a better solution.
I think it needs to work like this:
- temporary references (for read/write syscalls and friends) get
retrieved via get_xip_page and returned again via to-be-invented
put_page/page_cache_release
- permanent references (for mapping to userland) get retrieved via
get_xip_page and don't get returned until unmap
- the device driver can access page->count via a helper function
provided by mm. This way, it can identify which pages are in use.
- In order to get references back, the device driver can call a
callback provided by the file system. A default implementation will go
to filemap_xip.c. This callback would use rmap to find all mappings,
and unmap the page via xip_file_unmap()[mm/filemap_xip.c].

The nice thing about this approach is: we use page->count and rmap,
both already exist and are perfectly suited for our purpose.
The downside: We need mem_map[] struct page entries behind all memory
segments. Nowerdays we can easily create those via vmem_map/sparsemem.

Opinions?


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