Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache
From: Matthew Wilcox <willy@infradead.org>
Date: 2021-03-21 10:54:59
Also in:
ceph-devel, linux-cifs, linux-fsdevel, linux-nfs, lkml
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 (local)
).
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.
quoted hunk ↗ jump to hunk
+++ 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); + /**