linux-kernel - Re: [PATCH 03/12] writeback: Factor should_writeback_folio() out of write_cache

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Date:   Tue, 27 Jun 2023 12:16:34 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.com>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 03/12] writeback: Factor should_writeback_folio() out of
 write_cache_pages()

On Mon, Jun 26, 2023 at 09:12:07PM -0700, Christoph Hellwig wrote:
> > +	if (folio_test_writeback(folio)) {
> > +		if (wbc->sync_mode != WB_SYNC_NONE)
> > +			folio_wait_writeback(folio);
> > +		else
> > +			return false;
> > +	}
> 
> Please reorder this to avoid the else and return earlier while you're
> at it:
> 
> 	if (folio_test_writeback(folio)) {
> 		if (wbc->sync_mode == WB_SYNC_NONE)
> 			return false;
> 		folio_wait_writeback(folio);
> 	}

Sure, that makes sense.

> (that's what actually got me started on my little cleanup spree while
> checking some details of the writeback waiting..)

This might be a good point to share that I'm considering (eventually)
not taking the folio lock here.

My plan looks something like this (not fully baked):

truncation (and similar) paths currently lock the folio,  They would both
lock the folio _and_ claim that they were doing writeback on the folio.

Filesystems would receive the folio from the writeback iterator with
the writeback flag already set.

This allows, eg, folio mapping/unmapping to take place completely
independent of writeback.  That seems like a good thing; I can't see
why the two should be connected.

> > +	BUG_ON(folio_test_writeback(folio));
> > +	if (!folio_clear_dirty_for_io(folio))
> > +		return false;
> > +
> > +	return true;
> 
> ..
> 
> 	return folio_clear_dirty_for_io(folio);
> 
> ?

I did consider that, but there's a nice symmetry to the code the way it's
currently written, and that took precedence in my mind over "fewer lines
of code".  There's nothing intrinsic about folio_clear_dirty_for_io()
being the last condition to be checked (is there?  We have to
redirty_for_io if we decide to not start writeback), so it seemed to
make sense to leave space to add more conditions.