On Wed, 19 Sep 2007 16:43:08 +0900 KAMEZAWA Hiroyuki
<[email protected]> wrote:
> A clarification of page <-> fs interface (page cache).
>
> At first, each FS has to access to struct page->mapping directly.
> But it's not just pointer. (we use special 1bit enconding for anon.)
>
> Although there is historical consensus that page->mapping points to its inode's
> address space, I think adding some neat helper functon is not bad.
>
> This patch adds page-cache.h which containes page<->address_space<->inode
> function which is required (used) by subsystems.
>
> Following functions are added
>
> * page_mapping_cache() ... returns address space if a page is page cache
> * page_mapping_anon() ... returns anon_vma if a page is anonymous page.
> * page_is_pagecache() ... returns true if a page is page-cache.
> * page_inode() ... returns inode which a page-cache belongs to.
> * is_page_consistent() ... returns true if a page is still valid page cache
>
> Followings are moved
> * page_mapping() ... returns swapper_space or address_space a page is on.
> (from mm.h)
> * page_index() ... returns position of a page in its inode
> (from mm.h)
> * remove_mapping() ... a safe routine to remove page->mapping from page.
> (from swap.h)
I have two other functions that might want integration with this scheme:
page_file_mapping() ... returns backing address space
page_file_index() ... returns the index therein
They are identical to page_mapping_cache() and page_index() for
page cache pages, but they also work on swap cache pages.
That is, for swapcache pages they return:
page_file_mapping:
page_swap_info(page)->swap_file->f_mapping
page_file_index:
swp_offset((swp_offset_t)page_private(page))
When a filesystem uses these functions instead of page->mapping and
page->index, it allows passing swap cache pages into the regular
filesystem read/write paths.
This is useful for things like swap over NFS, where swap is backed by a
swapfile on a 'regular' filesystem.
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