Re: [RFC][patch 0/2] mm: remove PageReserved

Daniel Phillips
2005-08-10T23:19:13Z

On Thursday 11 August 2005 00:27, David Howells wrote:
> What happens is this:
>
> (1) readpage() is issued against NFS (for example).
>
> (2) NFS consults the local cache, and finds the page isn't available
> there.
>
> (3) NFS reads the page from the server.
>
> (4) NFS sets PG_fs_misc and tells the cache to store the page.
>
> (5) NFS sets PG_uptodate and unlocks the page.
>
> Some time later, the cache finishes writing the page to disk:
>
> (6) The cache calls NFS to say that it's finished writing the page.
>
> (7) NFS calls end_page_fs_misc() - which clears PG_fs_misc - to indicate
> to any waiters that the page can now be written to.
>
> Now: any PTEs set up to point to this page start life read-only. If they're
> part of a shared-writable mapping, then the MMU will generate a WP fault
> when someone attempts to write to the page through that mapping:
>
> (a) do_wp_page() gets called.
>
> (b) do_wp_page() sees that the page's host has registered an interest in
> knowing that the page is becoming writable:
>
> vm_operations_struct::page_mkwrite()
>
> (c) do_wp_page() calls out to the filesystem.
>
> (d) NFS sees the page is wanting to become writable and waits for the
> PG_fs_misc flag to become cleared.
>
> (e) NFS returns to the caller and things proceed as normal.
>
> Doing this permits the cache state to be more predictable in the event of
> power loss because we know that userspace won't have scribbled on this page
> whilst the cache was trying to write it to disk.

Hi David,

To be honest I'm having some trouble following this through logically. I'll
read through a few more times and see if that fixes the problem. This seems
cluster-related, so I have an interest.

Who is using this interface?

Regards,

Daniel