linux-kernel - Re: [PATCH v2 2/2] netfs: Fix dodgy maths

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Message-ID: <084d78a4-6052-f2ec-72f2-af9c4979f5dc@linux.alibaba.com>
Date:   Tue, 8 Nov 2022 13:54:47 +0800
From:   JeffleXu <jefflexu@...ux.alibaba.com>
To:     David Howells <dhowells@...hat.com>, willy@...radead.org
Cc:     Jeff Layton <jlayton@...nel.org>, linux-cachefs@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] netfs: Fix dodgy maths

On 11/5/22 12:38 AM, David Howells wrote:
> Fix the dodgy maths in netfs_rreq_unlock_folios().  start_page could be
> inside the folio, in which case the calculation of pgpos will be come up
> with a negative number (though for the moment rreq->start is rounded down
> earlier and folios would have to get merged whilst locked)

Hi, the patch itself seems fine. Just some questions about the scenario.

1. "start_page could be inside the folio" Is that because
.expand_readahead() called from netfs_readahead()? Since otherwise,
req-start is always aligned to the folio boundary.

2. If start_page is indeed inside the folio, then only the trailing part
of the first folio can be covered by the request, and this folio will be
marked with uptodate, though the beginning part of the folio may have
not been read from the cache. Is that expected? Or correct me if I'm wrong.

-- 
Thanks,
Jingbo