Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 10, 2021 at 04:54:49PM +0000, David Howells wrote:
> Add a function, unlock_page_private_2(), to unlock PG_private_2 analogous
> to that of PG_lock.  Add a kerneldoc banner to that indicating the example
> usage case.

One of the things which confused me about this was ... where's the other
side?  Where's lock_page_private_2()?  Then I found this:

#ifdef CONFIG_AFS_FSCACHE
        if (PageFsCache(page) &&
            wait_on_page_bit_killable(page, PG_fscache) < 0)
                return VM_FAULT_RETRY;
#endif

Please respect the comment!

/*
 * This is exported only for wait_on_page_locked/wait_on_page_writeback, etc.,
 * and should not be used directly.
 */
extern void wait_on_page_bit(struct page *page, int bit_nr);
extern int wait_on_page_bit_killable(struct page *page, int bit_nr);

I think we need the exported API to be wait_on_page_private_2(), and
AFS needs to not tinker in the guts of filemap.  Otherwise you miss
out on bugfixes like c2407cf7d22d0c0d94cf20342b3b8f06f1d904e7 (see also
https://lore.kernel.org/linux-fsdevel/20210320054104.1300774-4-willy@infradead.org/T/#u
).

That also brings up that there is no set_page_private_2().  I think
that's OK -- you only set PageFsCache() immediately after reading the
page from the server.  But I feel this "unlock_page_private_2" is actually
"clear_page_private_2" -- ie it's equivalent to writeback, not to lock.

> +++ b/mm/filemap.c
> @@ -1432,6 +1432,26 @@ void unlock_page(struct page *page)
>  }
>  EXPORT_SYMBOL(unlock_page);
>  
> +/**
> + * unlock_page_private_2 - Unlock a page that's locked with PG_private_2
> + * @page: The page
> + *
> + * Unlocks a page that's locked with PG_private_2 and wakes up sleepers in
> + * wait_on_page_private_2().
> + *
> + * This is, for example, used when a netfs page is being written to a local
> + * disk cache, thereby allowing writes to the cache for the same page to be
> + * serialised.
> + */
> +void unlock_page_private_2(struct page *page)
> +{
> +	page = compound_head(page);
> +	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
> +	clear_bit_unlock(PG_private_2, &page->flags);
> +	wake_up_page_bit(page, PG_private_2);
> +}
> +EXPORT_SYMBOL(unlock_page_private_2);
> +
>  /**