linux-ext4 - Re: [RFC 2/3] ext2: Convert ext2 regular file buffered I/O to use iomap

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Date: Thu, 30 Nov 2023 16:03:37 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Ritesh Harjani <ritesh.list@...il.com>
Cc: Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 2/3] ext2: Convert ext2 regular file buffered I/O to use
 iomap

On Thu, Nov 30, 2023 at 09:20:41PM +0530, Ritesh Harjani wrote:
> OR, one other approach to this can be... 
> 
> IIUC, for a new block mapping for the same folio, the folio can be made to
> get invalidated once in between right? So should this be handled in folio
> cache somehow to know if for a given folio the underlying mapping might
> have changed for which iomap should again query  ->map_blocks() ? 
> ... can that also help against unnecessary re-validations against cached
> block mappings?

That's pretty much exactly what buffer heads do -- we'd see the mapped
bit was now clear.  Maybe we could have a bit in the iomap_folio_state
that is set once a map has been returned for this folio.  On the one
hand, this is exactly against the direction of iomap; to divorce the
filesystem state from the page cache state.  On the other hand, maybe
that wasn't a great idea and deserves to be reexamined.