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From: David Howells <dhowells@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: dhowells@redhat.com, Trond Myklebust <trondmy@hammerspace.com>,
	Anna Schumaker <anna.schumaker@netapp.com>,
	Steve French <sfrench@samba.org>,
	Dominique Martinet <asmadeus@codewreck.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-mm@kvack.org, linux-cachefs@redhat.com,
	linux-afs@lists.infradead.org, linux-nfs@vger.kernel.org,
	linux-cifs@vger.kernel.org, ceph-devel@vger.kernel.org,
	v9fs-developer@lists.sourceforge.net,
	linux-fsdevel@vger.kernel.org, Jeff Layton <jlayton@redhat.com>,
	David Wysochanski <dwysocha@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache
Date: Tue, 23 Mar 2021 14:16:28 +0000	[thread overview]
Message-ID: <2503810.1616508988@warthog.procyon.org.uk> (raw)
In-Reply-To: <20210323135116.GF1719932@casper.infradead.org>

Matthew Wilcox <willy@infradead.org> wrote:

> 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.

Do I need to?  Doesn't VM_FAULT_LOCKED indicate that to the caller?  Or is it
impermissible to do it like that?

> 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?

I'm waiting both before and after.  If I wait before, write() can go and
trample over the page between PG_writeback/PG_fscache being cleared and us
getting the lock here.  Probably I should only be waiting after locking the
page.

> 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.

Um.  I do want to add disconnected operation in the future and cache
encryption, but, as things currently stand, it isn't necessary because the
cache object is marked "in use" and will be discarded on rebinding after a
power loss or crash if it's still marked when it's opened again.

Also, the thought has occurred to me that I can make use of reflink copy to
handle the caching of local modifications to cached files, in which case I'd
rather have a clean copy to link from.

David


  parent reply	other threads:[~2021-03-23 14:18 UTC|newest]

Thread overview: 49+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-10 16:54 [PATCH v4 00/28] Network fs helper library & fscache kiocb API David Howells
2021-03-10 16:54 ` [PATCH v4 01/28] iov_iter: Add ITER_XARRAY David Howells
2021-03-10 16:54 ` [PATCH v4 02/28] mm: Add an unlock function for PG_private_2/PG_fscache David Howells
2021-03-16 19:07   ` Matthew Wilcox
2021-03-17  0:43     ` Linus Torvalds
2021-03-17  0:43       ` Linus Torvalds
2021-03-17  2:12       ` Josef Bacik
2021-03-17  2:35         ` Linus Torvalds
2021-03-17  2:35           ` Linus Torvalds
2021-03-17  9:04         ` David Howells
2021-03-17 10:53         ` David Howells
2021-03-16 20:38   ` David Howells
2021-03-16 23:32     ` Matthew Wilcox
2021-03-21 10:53   ` Matthew Wilcox
2021-03-22 10:56   ` David Howells
2021-03-23 13:17   ` David Howells
2021-03-23 13:51     ` Matthew Wilcox
2021-03-23 14:16     ` David Howells [this message]
2021-03-23 22:06     ` David Howells
2021-03-23 22:06       ` David Howells
2021-03-10 16:55 ` [PATCH v4 03/28] mm: Implement readahead_control pageset expansion David Howells
2021-03-10 16:55 ` [PATCH v4 04/28] netfs: Make a netfs helper module David Howells
2021-03-10 16:55 ` [PATCH v4 05/28] netfs: Documentation for helper library David Howells
2021-03-10 16:55 ` [PATCH v4 06/28] netfs, mm: Move PG_fscache helper funcs to linux/netfs.h David Howells
2021-03-10 16:56 ` [PATCH v4 07/28] netfs, mm: Add unlock_page_fscache() and wait_on_page_fscache() David Howells
2021-03-10 16:56 ` [PATCH v4 08/28] netfs: Provide readahead and readpage netfs helpers David Howells
2021-03-21  1:42   ` Matthew Wilcox
2021-03-22 17:13   ` David Howells
2021-03-10 16:56 ` [PATCH v4 09/28] netfs: Add tracepoints David Howells
2021-03-10 16:56 ` [PATCH v4 10/28] netfs: Gather stats David Howells
2021-03-10 16:56 ` [PATCH v4 11/28] netfs: Add write_begin helper David Howells
2021-03-10 16:57 ` [PATCH v4 12/28] netfs: Define an interface to talk to a cache David Howells
2021-03-10 16:57 ` [PATCH v4 13/28] netfs: Hold a ref on a page when PG_private_2 is set David Howells
2021-03-10 16:57 ` [PATCH v4 14/28] fscache, cachefiles: Add alternate API to use kiocb for read/write to cache David Howells
2021-03-10 16:57 ` [PATCH v4 15/28] afs: Disable use of the fscache I/O routines David Howells
2021-03-10 16:58 ` [PATCH v4 16/28] afs: Pass page into dirty region helpers to provide THP size David Howells
2021-03-10 16:58 ` [PATCH v4 17/28] afs: Print the operation debug_id when logging an unexpected data version David Howells
2021-03-10 16:58 ` [PATCH v4 18/28] afs: Move key to afs_read struct David Howells
2021-03-10 16:58 ` [PATCH v4 19/28] afs: Don't truncate iter during data fetch David Howells
2021-03-10 16:58 ` [PATCH v4 20/28] afs: Log remote unmarshalling errors David Howells
2021-03-10 16:59 ` [PATCH v4 21/28] afs: Set up the iov_iter before calling afs_extract_data() David Howells
2021-03-10 16:59 ` [PATCH v4 22/28] afs: Use ITER_XARRAY for writing David Howells
2021-03-10 16:59 ` [PATCH v4 23/28] afs: Wait on PG_fscache before modifying/releasing a page David Howells
2021-03-10 16:59 ` [PATCH v4 24/28] afs: Extract writeback extension into its own function David Howells
2021-03-10 16:59 ` [PATCH v4 25/28] afs: Prepare for use of THPs David Howells
2021-03-10 17:00 ` [PATCH v4 26/28] afs: Use the fs operation ops to handle FetchData completion David Howells
2021-03-10 17:00 ` [PATCH v4 27/28] afs: Use new fscache read helper API David Howells
2021-03-10 17:00 ` [PATCH v4 28/28] afs: Use the fscache_write_begin() helper David Howells
2021-03-16 11:34 ` [PATCH v4 08/28] netfs: Provide readahead and readpage netfs helpers David Howells

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