linux-kernel - Re: [RFC] xfs: fake fallocate success for always CoW inodes

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Message-ID: <lhubjlaz08f.fsf@oldenburg.str.redhat.com>
Date: Mon, 10 Nov 2025 10:49:04 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Christoph Hellwig <hch@....de>
Cc: Matthew Wilcox <willy@...radead.org>,  Hans Holmberg
 <hans.holmberg@....com>,  linux-xfs@...r.kernel.org,  Carlos Maiolino
 <cem@...nel.org>,  Dave Chinner <david@...morbit.com>,  "Darrick J . Wong"
 <djwong@...nel.org>,  linux-fsdevel@...r.kernel.org,
  linux-kernel@...r.kernel.org,  libc-alpha@...rceware.org
Subject: Re: [RFC] xfs: fake fallocate success for always CoW inodes

* Christoph Hellwig:

>> Maybe add two flags, one for the ftruncate replacement, and one that
>> instructs the file system that the range will be used with mmap soon?
>> I expect this could be useful information to the file system.  We
>> wouldn't use it in posix_fallocate, but applications calling fallocate
>> directly might.
>
> What do you think "to be used with mmap" flag could be useful for
> in the file system?  For file systems mmap I/O isn't very different
> from other use cases.

I'm not a file system developer. 8-)

The original concern was about a large file download tool that didn't
download in sequence.  It wrote to a memory mapping directly, in
somewhat random order.  And was observed to cause truly bad
fragmentation in practice.  Maybe this something for posix_fadvise.

Thanks,
Florian