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Date: Tue, 19 Mar 2024 17:59:42 +0100
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Subject: Re: [RFC PATCH] mm: Replace ->launder_folio() with flush and wait
To: Miklos Szeredi <miklos@szeredi.hu>, David Howells <dhowells@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>,
 Trond Myklebust <trond.myklebust@hammerspace.com>,
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From: Bernd Schubert <bernd.schubert@fastmail.fm>
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On 3/19/24 17:40, Miklos Szeredi wrote:
> On Tue, 19 Mar 2024 at 17:13, Miklos Szeredi <miklos@szeredi.hu> wrote:
>>
>> On Tue, 19 Mar 2024 at 15:15, David Howells <dhowells@redhat.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