linux-kernel - Re: [PATCH] writeback: Account the number of pages written back

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Message-ID: <ZKIuu6uQQJIQE640@casper.infradead.org>
Date:   Mon, 3 Jul 2023 03:13:15 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] writeback: Account the number of pages written back

On Sun, Jul 02, 2023 at 01:06:15PM -0700, Andrew Morton wrote:
> On Wed, 28 Jun 2023 19:55:48 +0100 "Matthew Wilcox (Oracle)" <willy@...radead.org> wrote:
> 
> > nr_to_write is a count of pages, so we need to decrease it by the number
> > of pages in the folio we just wrote, not by 1.  Most callers specify
> > either LONG_MAX or 1, so are unaffected, but writeback_sb_inodes()
> > might end up writing 512x as many pages as it asked for.
> 
> 512 is a big number,  Should we backport this?

I'm really not sure.  Maybe?  I'm hoping one of the bots comes up with a
meaningful performance change as a result of this patch and we find out.

> > Fixes: 793917d997df ("mm/readahead: Add large folio readahead")
> 
> I'm not seeing how a readahead change messed up writeback accounting?

That was the first patch which allowed large folios to be added to the
page cache.  Until that point, this was latent.  We could probably argue
for one of a dozen other commits around the same time.