linux-kernel - Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG

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Date:   Tue, 23 Mar 2021 13:51:16 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        linux-cachefs@...hat.com, linux-afs@...ts.infradead.org,
        linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
        ceph-devel@...r.kernel.org, v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, Jeff Layton <jlayton@...hat.com>,
        David Wysochanski <dwysocha@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 02/28] mm: Add an unlock function for
 PG_private_2/PG_fscache

On Tue, Mar 23, 2021 at 01:17:20PM +0000, David Howells wrote:
> +++ b/fs/afs/write.c
> @@ -846,7 +846,7 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
>  	 */
>  #ifdef CONFIG_AFS_FSCACHE
>  	if (PageFsCache(page) &&
> -	    wait_on_page_bit_killable(page, PG_fscache) < 0)
> +	    wait_on_page_fscache_killable(page) < 0)
>  		return VM_FAULT_RETRY;
>  #endif
>  
> @@ -861,7 +861,8 @@ vm_fault_t afs_page_mkwrite(struct vm_fault *vmf)
>  	 * details the portion of the page we need to write back and we might
>  	 * need to redirty the page if there's a problem.
>  	 */
> -	wait_on_page_writeback(page);
> +	if (wait_on_page_writeback_killable(page) < 0)
> +		return VM_FAULT_RETRY | VM_FAULT_LOCKED;

You forgot to unlock the page.  Also, if you're waiting killably here,
do you need to wait before you get the page lock?  Ditto for waiting on
fscache -- do you want to do that before or after you get the page lock?

Also, I never quite understood why you needed to wait for fscache
writes to finish before allowing the page to be dirtied.  Is this a
wait_for_stable_page() kind of situation, where the cache might be
calculating a checksum on it?  Because as far as I can tell, once the
page is dirty in RAM, the contents of the on-disk cache are irrelevant ...
unless they're part of a RAID 5 checksum kind of situation.

I didn't spot any other problems ...