linux-kernel - Re: [RFC PATCH] mm: Replace ->launder

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Message-ID: <63e67db9-7425-4928-afb2-cbe7cc6232bb@fastmail.fm>
Date: Tue, 19 Mar 2024 17:59:42 +0100
From: Bernd Schubert <bernd.schubert@...tmail.fm>
To: Miklos Szeredi <miklos@...redi.hu>, David Howells <dhowells@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,
 Trond Myklebust <trond.myklebust@...merspace.com>,
 Christoph Hellwig <hch@....de>, Andrew Morton <akpm@...ux-foundation.org>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jeff Layton <jlayton@...nel.org>,
 linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, netfs@...ts.linux.dev,
 v9fs@...ts.linux.dev, linux-afs@...ts.infradead.org,
 ceph-devel@...r.kernel.org, linux-cifs@...r.kernel.org,
 linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm: Replace ->launder_folio() with flush and wait



On 3/19/24 17:40, Miklos Szeredi wrote:
> On Tue, 19 Mar 2024 at 17:13, Miklos Szeredi <miklos@...redi.hu> wrote:
>>
>> On Tue, 19 Mar 2024 at 15:15, David Howells <dhowells@...hat.com> wrote:
>>
>>> What particular usage case of invalidate_inode_pages2() are you thinking of?
>>
>> FUSE_NOTIFY_INVAL_INODE will trigger invalidate_inode_pages2_range()
>> to clean up the cache.
>>
>> The server is free to discard writes resulting from this invalidation
>> and delay reads in the region until the invalidation finishes.  This
>> would no longer work with your change, since the mapping could
>> silently be reinstated between the writeback and the removal from the
>> cache due to the page being unlocked/relocked.
> 
> This would also matter if a distributed filesystem wanted to implement
> coherence even if there are mmaps.   I.e. a client could get exclusive
> access to a region by issuing FUSE_NOTIFY_INVAL_INODE on all other
> clients and blocking reads.  With your change this would fail.
> 
> Again, this is purely theoretical, and without a way to differentiate
> between the read-only and write cases it has limited usefulness.
> Adding leases to fuse (which I plan to do) would make this much more
> useful.

Thanks Miklos! Fyi, we are actually planning to extend fuse
notifications from inode to page ranges.


Thanks,
Bernd