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SubjectRe: [PATCH 2.6.21] cramfs: add cramfs Linear XIP
Jörn Engel wrote:
> Whenever writes/erases to the device happen, the device driver would
> need to call a function like
>
> /**
> * unmap_page_range - remove all mapping to the given range of an address space
> * @mapping - the address space in question
> * @start_index - index of the first page in the range
> * @no_pages - number of pages to get unmapped
> *
> * Returns 0 on success or a negative errno value.
> */
> int unmap_page_range(struct address_space *mapping, loff_t start_index,
> loff_t no_pages);
>
> or implement something equivalent itself. Your filesystem callback
> looks like it would be just that, although I may be misreading you.
No. That's how the callback could look alike.

> Either that or using standard mtd->read() and mtd->write() calls. I see
> some advantages to mtd->write() in particular, as the device driver
> needs some notification to trigger unmap_page_range() before the actual
> write and chip state transitions happen. mtd->write() seems much easier
> than something like
>
> mtd->pre_write()
> get_xip_page()
> ...
> put_page()
> mtd->post_write()
>
> If get_xip_page() only has userland consumers all the locking can be
> kept inside device drivers.
Hmmh. We won't need mtd->pre_write(), because the file system's
get_xip_page aop will have to ask mtd for the address anyway similar
to the way ext2_get_xip_page does call bdev_ops->direct_access.

If that call would pass the information whether the future access is
read-only or read+write, the device driver could do its housekeeping.

I think we should also cover put_page() plus mtd->post_write() inside
a put_xip_page() address space operation provided by the fs. This way
calls are balanced, and we have stuff in a single place rather then
duplicating code. The fs could rely on a generic implementation in
filemap_xip in case it does'nt need to do its own magic here.

>> - the device driver can access page->count via a helper function
>> provided by mm. This way, it can identify which pages are in use.
>
> One of us is confused here. The driver would have to check page->count
> for a large range of pages, usually the whole chip. And it would have
> to tear down every single mapping before starting to write. Is that
> possible and desirable to do with page->count? Unsure.
That point was indeed confusing. Let my try again:

The only way, how an initial reference to a page can be retrieved, is
from the driver by using a bdev_ops->direct_access alike call. The
driver has to be able to check, if all references have been returned.
That would be done in three steps:
1. stop handing out new references
2. use the unmap_page_range callback to get references for page table
entries back
3. check if other (temporary) references are still handed out, wait
until they get returned.

Using a helper function to look into page->count is a possible
implementation for #3. And because the page->count cannot get changed
while noone has a reference, I don't see a race condition in looping
over all pages and checking page->count. This implementation has an
advantage, if the device driver wants to make sure that a small page
range is not accessed.

If the device driver needs to ensure that there are no references to
any page on the enitre flash media, it might use a global counter to
count the sum of references and save looping over all pages.

so long,
Carsten

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