linux-kernel - Re: The mapping->invalidate

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Message-ID: <20240828152638.iv7v5rj23n7mi73h@quack3>
Date: Wed, 28 Aug 2024 17:26:38 +0200
From: Jan Kara <jack@...e.cz>
To: David Howells <dhowells@...hat.com>
Cc: Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
	Steve French <sfrench@...ba.org>, netfs@...ts.linux.dev,
	linux-cifs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: The mapping->invalidate_lock, copy-offload and cifs

Hi David!

On Tue 27-08-24 15:46:55, David Howells wrote:
> I'm looking at trying to fix cifs_file_copychunk_range().  Currently, it
> invalidates the destination range, apart from a partial folio at either end
> which will be flushed, and then tries the copy.  But if the copy fails or can
> only be partially completed (eg. ENOSPC), we lose any data in the destination
> region, so I think it needs to be flushed and invalidated rather than just
> being invalidated.
> 
> Now, we have filemap_invalidate_inode() which I can use to flush back and
> invalidate the folios under the invalidate_lock (thereby avoiding the need for
> launder_folio).  However, that doesn't prevent mmap from reinstating the
> destination folios with modifications whilst the copy is ongoing the moment
> the invalidate_lock is dropped.
> 
> Question is: would it be reasonable to do the copy offload whilst holding the
> invalidate_lock for the duration?

FWIW yes, I'd expect cifs_file_copychunk_range() to take invalidate_lock on
the target file to avoid possible races with page faults. We do it this
already for similar operations such as reflink or various fallocate
operations...

								Honza

> 
> Thanks,
> David
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR